The Disability Support Act is intended to integrate welfare services for those who are physically, intellectually, or mentally disabled and promote the independence and social participation. However, the Act has been the center of intense controversy since its implementation in 2005. A number of important issues on implementing the Act in psychiatric medicine have been raised, and the validity of the for evaluating algorithm the degree of disability in those who are mentally disabled is considered to be of the utmost importance. We investigated the validity of the algorithm for evaluating the degree of disability in 2,200 patients who were institutionalized in long-term psychiatric care facilities in Hiroshima. The discordance rate between the primary evaluation by computer program and secondary evaluation by specialists' review was 50.9%. A wide disparity in evaluating the degree of disability was observed between that by secondary evaluation by specialists' review and that by evaluation of IADL. This disparity was more prominent in those who were more severely disabled. Further studies are needed to develop a new evaluation method to address the psychiatric signs and symptoms, and cognitive deficit of the mentally disabled.