PURPOSE: To enable the management of an underwear washing business system which have been conventionally impossible in view of a problem of privacy and complications of sorting or the like.

CONSTITUTION: A customer who have bought an empty back assembly comprising a water-permeable and translucent outer bag and a water-decomposable inner bag, stores underwares or the like in the inner bag and sealed the latter, and request for washing the underweares packed in the bag assembly as they are, and accordingly, the underwear are washed, and are then subjected to germ-resistant treatment and deodorization, and are then returned to the customer with the back assembly being held to be closed through the exchange with an identification. Further, the fees for washing service is included in the price of the back assembly.