Abstract: A tandem-type vacuum booster having a partition plate providing front and rear chambers with a booster piston and diaphragm in each chamber. A piston boss is slidably supported on the partition plate and a connecting cylinder is formed on the front chamber booster piston. A press plate is provided on the piston boss and the rear booster piston and an end plate of the connecting cylinder are coupled to the piston boss by the press plate and a plurality of bolts disposed around the piston boss with the rear booster piston and diaphragm clamped between the press plate and piston boss. The front ends of the bolts are disposed to come closer to the piston boss axis than the rear ends. The bolt heads are also received in recesses provided in the press plate to prevent relative rotation between the bolts and press plate.