This report introduces development of a drainage system equipped with special fitting inside buildings and verification of permissible drain flow rate in the system by adapting to an existing building design as follows. (1)The basic performance of the fitting is obtained making maximum negative pressure in drain pipe and air velocity in stack vent pipe as indexes through an experiment at 10 floors high tower. (2)The permissible drain flow rate is defined by giving steady and fixture drain flows from multiple floors. (3)The verification test of permissible drain flow rate is conductted by using the drainage system in super-high rise apartment.