One goal of the policy was to rehabilitate criminals (they earned small salaries from their labor, and those who repented of their crimes would be released. Convicts on Sadoga-shima Island, much like those at the work-houses of Tsukudajima Island, were given one type of labor to perform, and so if they showed contrition they would receive a tiny sum as savings and, after many years, would be allowed to return to their hometowns), but drainage work was extremely heavy labor, and drainage laborers were so cruelly exploited that it was said they could not survive for more than three years.
犯罪者の更生という目的もあった（作業に応じて小遣銭が支給され、改悛した者は釈放された。