PURPOSE: To reduce an oil pressure drop in a wheel cylinder in a brake oil pressure generating device in the case where a force boosting ratio in a booster is smaller than a set force intensifying factor.

CONSTITUTION: When a booster 12 is normal, a changing-over valve device 48 does not operate, and a master cylinder 10 is kept in the first state. Brake oil in a pressurizing chamber 22 is pressurized by a large diameter part 62 of a first pressurizing piston 18, and the oil pressure is transmitted to a wheel cylinder 28. When the force boosting ratio has been decreased to the value smaller than a set force intensifying factor, a changing-over valve 48 operates, and the master cylinder 10 is changed over to the second state. The oil pressure in the pressurizing chamber 22 is increased by the small diameter part 64 of the first pressurizing piston 18, and this oil pressure is transmitted to the wheel cylinder 28. In the latter case, a higher oil pressure than in the former case for the equal output of the booster 12 is generated. For this reason, decrease of the oil pressure in the wheel cylinder 28 caused by the breakage of the booster 12 is reduced.