Japanese adjectives have undergone many phonological, morphological, and syntactic changes from ancient to modern Japanese. In this paper, we analyze these changes in detail and clarify the evolution of adjectives from ancient to modern Japanese. We consider how Japanese adjectives have undergone morphological and syntactic changes as the language has developed. Adjectives in Old Japanese are classified as adjectives (-i adjectives), while from Early Middle Japanese the category of adjectival verbs (-na adjectives) exists. -i adjectives have a similar conjugation pattern to verbs, and use the endings ki and shiki. In contrast, adjectival verbs function as adjectives by adding the copula nari to the noun. In Middle Japanese, adjectives show many phonological and morphological changes. During this period, -i adjective endings were simplified and/or tended to disappear. This change shows the process of adjective conjugation patterns moving to more modern forms. Furthermore, the use of auxiliary verbs increased and -i adjectives were integrated into the verb system. The use of auxiliary verbs tari and beshi became common, and the past tense and negative forms of -i adjectives were further simplified. The use of the copula nari was also standardized for adjective verbs and became more established. In the later period of Middle Japanese (12th to 16th century), -i adjective conjugation was further simplified and became closer to modern Japanese. The past tense and negative forms of adjectives changed to more regular forms. During this period, adjectives became more morphologically unified. In addition, the copula nari changed in adjectival verbs. Nari changed to forms such as de aru and ni and their use in sentences became more evident. This change refined the grammatical role of adjectival verbs. For example, the expression shizuka de aru became common, and the function of adjectives became more evident. During the transition from Early Modern Japanese (17th to 19th centuries) to Modern Japanese (the end of the 19th century to the present), the conjugation patterns of -i adjectives became more regular and simplified, and the foundation of modern Japanese was established. Adjective conjugation was basically unified, and the past tense, negative form, and conjunctive form were standardized. The use of adjectival verbs was also established in modern Japanese, and the use of copulas was fixed as da and de aru. This simplified the conjugation and usage of adjectives very regularly, completing the adjective and adjectival verb system of modern Japanese. In the study on corpus analysis, adjectives are analyzed using data from the CHJ and BCCWJ corpora. The 20 most common words among Japanese adjectives and adjectival verbs in each historical period are selected and analyzed based on the historical and thematic context of the period. Because the size of the corpus, i.e. the amount of data available for each period, varies, these adjectives and adjectival verbs are compared using the „relative frequency“ method. The number of adjectives is seen to increase in certain periods, such as the Heian and Meiji periods. The largest increase was seen in the period 1971-2008, during which the number of adjectives and adjectival verbs increased almost 100-fold. The number of adjectives appears to have increased significantly since the Nara period, due to improvements in the quality of life, technological advances, and the arrival of new ideas and literary genres. In the Nara period, there were a total of 4,178, of which 3,371 were adjectives and 407 were adjectival verbs. Meanwhile, from the Shōwa period to the current Heisei period, there are 1,588,266 adjectives and 1,314,005 adjectival verbs. The number of adjectives has increased relatively year by year, but a decrease can be seen in the Muromachi period. The ratio of adjectives to adjectival verbs was very similar until the Meiji period. After the Meiji period, the number of adjectival verbs becomes comparable to that of adjectives. This proportional increase can be attributed to various factors. In the Heian and Meiji periods, when the value of Japanese culture was high in literary works, adjectival verbs (mainly loanwords) accounted for a smaller proportion. Furthermore, there are many synonyms of adjectival verbs in the form of adjectives, and it is possible that there was a tendency of using adjectives in works. At the beginning of the study (Nara and Heian), the changes in the most common adjectives were very large, but in later periods, the language was more specifically formed and almost the same adjectives appeared most frequently. At the same time, adjectives such as imiji-, kuchioshi-, and iyataka were either completely lost during the period or replaced by adjectives with similar meanings. Among adjectives, nai and yoi are the most frequently used adjectival forms throughout Japanese literature.