It is said that auxiliary verb ‘shimau’ has aspectual and modal meaning. It presents an aspectual meaning ‘perfect’ and in a lot of cases it also presents a negative evaluation of the subject matter by the speaker. ‘Shimau’ aslo seems to have two cotradictory meanings. It cancels and strengthens the volitionality of the verb it follows. In this article I tried to find out an basic meaning of ‘shimau’ that can be common to these two contradictory meanings. According to previous studies shimau’s modal meaning is “Things happen out of control by the speaker”. Within this frame shimau can cancel the volitionality of the verb only when subject of the verb is the speaker. But this definition of the modal meaning of shimau can not explain how shimau can also strengthen the volitionality of the verb. By examining the usages in which shimau seems to strengthen the volitionality of the verb, it is found out that the reinforcement of volition is presented by other elemnts but not by shimau itself. The surplus meaning added by auxiliary verb shimau is the action the concerned verb or verb phrase presents is unnatural and not along the way things go on by the speaker’s point of view. Shimau’s aspectual meaning ‘perfect’ means the action or event reaches the endpoint. In modal usage of shimau this endpoint seems to be more abstract. The endpoint is not the end of action or event itself but a borderline of situation which the speaker defines as natural or desirable.