A pressure reducer includes a piston slidably accommodated in a cylinder to define a decompression chamber and a pressure adjusting chamber. A seal member includes an elastic member and a sliding portion. The sliding portion is pressed by the elastic member such that the sliding portion slides on an inner circumferential surface of the cylinder. An annular member is located on an outer circumferential surface of the piston and between the seal member and the pressure adjusting surface. The annular member is pressed against the inner circumferential surface of the cylinder with a force that is less than a force with which the sliding portion is pressed against the inner circumferential surface of the cylinder. Thus, the pressure reducer prevents the piston from being moved by an excessive amount with highly accurately adjusting pressure.