Roughly 30-50% of clients reported to dropout from mental health treatment, and the dropout brought\a large loss to both therapist and client. Nevertheless, only little study has been made for the dropout\in mental health in Japan. Here this article reviewed the dropout regarding (a) definition of dropout, (b)\rate of dropout, (c) factor of dropout, (d) consideration in studying of dropout, (e) prevention measures\of a dropout. It was found that we need to refine the definition of the dropout which tended to fluctuate\among the studies. It was also found that most of the recent literature examined the factor of dropout\from user's demographic data. However, we rather have to accumulate the finding about how we could\make the relation better than finding clients’ factor, as only the therapist factor is changeable.