Housing supply systems have a close relationship to the characteristics of residents, especially in the sub-market of public housing. Because there are strict regulations for applicants for public housing, local governments mainly select tenants in regard to their income conditions. This study aims to examine the change in characteristics of residents in public housing and to clarify the cause of this transformation process. In the built-up area, rapid aging and: decrease in household sizes 1n public houses may cause serious social: problems. In the suburban area, aging and: decreasing household members in: public housing did not become prominent in any of the public housing types. Most young families in; public houses in suburban area moved out when they; aged, and elders without their own transportation tend to avoid inconvenient suburban public housing. Thus the Public Housing Act transformed the structures of dwellers' characteristics. For welfare purposes, the Public Housing Act gives priority to lower income households, for example elders. The Public Housing Act induced the aging of residents. Consequently, the welfare for economically weaker households will strengthen the housing trap.