To provide the structure of a seal member capable of preventing the generation of a cut of the seal member into a sliding member and restricting the sliding resistance of the sliding member when the sliding member moves inside of a pressure chamber.

In this pressure generating device 1, the seal member 4 is arranged between a pressure chamber 10 formed inside of a housing and a port 11 communicating with the pressure chamber 10, and a piston 6 moves in the pressure chamber to cut the communication between the pressure chamber 10 and the port 11 with the seal member 4 having a base part 4a, and inner peripheral rib 4b and an outer peripheral rib 4c. Shape of the seal member 4 is formed into a shape having a tapered surface 4d to be widened outward in the radial direction at a rear end of the inner peripheral rib 4b to be slid for contact with the piston 6 within a range of a width of the base part 4a in the axial direction, in which the piston 6 slides.