A suck-back valve includes a piston and a valve body linked via a shaft portion. An open/close valve, operated by fluid pressure, is provided with a suck-back function. A suck-back chamber formed in an internal channel of the open/close valve and an actuator that operates the piston are separated by a diaphragm linked to and operating with the shaft portion. The shaft portion includes a valve-body-side shaft portion and a piston-side shaft portion. The piston-side shaft portion is loosely fitted to the valve-body-side shaft portion, allowing for independent movement in an axial direction. The closing operation of the valve body involves a two-step operation where, first, the shaft portion, along with the piston and the diaphragm, moves to the fully closed position of the valve body and where, second, the piston-side shaft portion moves with the piston and the diaphragm to increase the volume of the suck-back chamber.