PURPOSE: To prevent the generation of the malfunction even if a liquid pressure sensor is installed in close to a booster device, by forming a throtle for drawing a flow passage in comparison with the flo passage area on a hydraulic pressure source side, in the passage leading from the liquid pressure sensor to a servopiston inside.

CONSTITUTION: The orifice 7500 of an orifice member 750 midway in a pressure intrducing passage has a diameter of 0.2mm or so, and the min. flow passage area is formed in the passage leading to the inside of a booster 100 through the pressure introducing passage from a hydraulic pressure source 300. Accordingly, even if the pressure lowering is generated inside the cylinder body 140 of the booster 100, by the sharp operation of the booster 100, the large lowering of the pressure at the part 136a is prevented, and the pressure becomes the value of the pressurized liquid inside an accumulator 320. Accordingly, so far as no pressure loss is generated on the liquid pressure source 300 side, a hydraulic pressure sensor 600 supends the switching operation, and malfunction can be surely prevented.