A pressurizing piston has a head section, which contacts an axial end face of a piezoelectric actuator to receive displacement of the actuator, and a piston wall section, which is formed in a cylindrical shape and can move in an axial direction in response to the movement of the head section. The head section and the piston wall section are combined such that relative displacement therebetween is possible in a radial direction. An outer sleeve slidably holding an outer periphery of the piston wall section restricts radial movement of the head section. When an expansion-contraction direction of the actuator inclines with respect to the axial direction, inclination of the piston wall section is inhibited by a radial deviation caused between the head section and the piston wall section. Thus, pinching of the piston wall section to the outer sleeve is inhibited.