PURPOSE: To test leakage for every group of a plurality of consumers immediately as required.

CONSTITUTION: In an underground service piping system comprising a supply side main piping 1 and a plurality of branch pipings 3 for a group of homes 2, a main plug 5 previously equipped with a test plug 6 is coupled between the main piping 1 and each branch piping 4. Upon elapse of a predetermined time after closing the main plug 5 and the main plug 4 on each home 2 side, variation in the pressure measured by a pressure gauge 8 coupled with the test plug 6 is compared.