A tubular member 18 is slidably fitted into a valve body, and is formed with a vacuum valve seat 21. The tubular member 18, a valve plunger 16 and the valve body 3 are maintained in their inoperative positions shown by abutment against a key member 13. A holder 14 is formed with a tapered surface 14a to assist in allowing an elastic deformation of a reaction disc 15. Immediately after the commencement of operation of a valve mechanism 7 as an input shaft 8 is driven forward, the input shaft 8 and the tubular member 18 are maintained in their inoperative positions until a rear end face of a variable pressure passage 28 abuts against the key member 13. The operating stroke of an input shaft 6 can be chosen to be less than the operating stroke of an output shaft 11 while preventing the jumping value from increasing to an excessively high value.