One embodiment of the present invention provides a brake hydraulic pressure generating device, including: a master cylinder, a stroke simulator and an on/off valve. The on/off valve opens/closes a branch hydraulic pressure passage, which branches from a main hydraulic pressure passage. The on/off valve includes a first valve mechanism including a first valve portion and a first valve seat, and a second valve mechanism including a second valve portion and a second valve seat. The first valve seat is formed in the second valve portion. A pressure receiving area in which brake hydraulic pressure acting from the stroke simulator side is received is set to be higher in the second valve portion than in the first valve portion.