PURPOSE: To provide gas-tight junction between a spacer and a connection member for a booster housing for a vehicle brake system without damaging the surface of the spacer.

CONSTITUTION: A booster housing has two housing bottom parts 26 respectively disposed on one side of a movable partition wall. Connection members 32 are gas-tightly caulked around one fixing hole 30 in at least one housing bottom part 26. A spacer 50 penetrates the movable partition wall to be extended between the two housing bottom parts 26. Each connection member 32 has a recess 40, and an end part 52 of the corresponding spacer 50 is inserted into it to be caulked. The booster housing is assembled in accordance with following processes: (i) the end part 52 of the spacer 50 is inserted into the recess 40 in each connection member 32, (ii) the connection member 32 is inserted into the fixing hole 30, and (iii) the connection member 32 is simultaneously caulked with the housing bottom part 26 and the spacer 50.