PURPOSE: To reduce a longitudinal size of a negative pressure booster, and eliminate effects of oscillation of an input lever.

CONSTITUTION: A control valve V actuated by relative motion of a piston boss 5 and a valve piston 14 is provided with a valve spring 24 to seat a first valve body 221 and a second valve body 222 on a first valve seat 211 and a second valve seat 212 respectively, and a return spring 26 to energize the valve piston 14 back. The return spring 26 is contained in a recessed part 5a formed in the piston boss 5, so as inner space of the piston boss 5 is used effectively to reduce the longitudinal size of a negative pressure booster. Because both the valve spring 24 and the return spring 26 are supported by the valve piston 14, effects of oscillation of an input lever 13 applied on the control valve V can be prevented.