The auxiliary verb beshi was originally conjugated with ku. However, during the Muromachi period, one begins to see examples of shiku conjugation appearing as well. In this paper we focus on the fact that in shiku conjugations, this change only occurs in the example of tsubei (originally tsu+beshi). We investigate the reasons for this by considering the dual meanings of beshi, as well as the historical change of its system of modality. The meaning of beshi is broadly divided into the two functions of (1) expressing the state or nature of something (an expression of state-of-affairs) and (2) expressing the operation of the speaker's judgment (an expression of epistemic modality). Tsubeshi and nubeshi have existed as exclusive forms of the 'state-of-affairs' meaning of beshi since the Heian period, whereas tsubeshi expressed 'possibility (dynamic possibility)', and complimentarily, nubeshi expressed 'probability (existential possibility)'. During the Muromachi period, from tsubei, tsubeshii developed, which was used to express a state in an evaluative manner. This can be interpreted as one phenomenon within the process of shiku-conjugated adjectives coming to be derived from ku-conjugated adjectives, a change that occurred during this period. Also, uzu assumed the 'epistemic' meaning of beshi, and it was beshi that was incorporated into the literary language. Tsubei, which expresses the 'state- of-affairs' meaning of beshi, remained in the spoken language longer than beshi. It is thought that tsubei remained in existence until the development and progression of expressions such as sona (written as sauna).