Suck-back valve

A suck-back valve comprising:

• a valve main body formed with a fluid flow passage for a fluid to flow therethrough, including an inlet flow passage and an outlet flow passage, and a suck-back chamber being in communication with the outlet flow passage;
• and a suck-back mechanism unit, used to suck back the fluid from the outlet flow passage by increasing the volume of the suck-back chamber,
• wherein the suck-back mechanism unit includes a suck-back drive unit housing having a suck-back cylinder chamber formed therein, a suck-back piston accommodated in the suck-back cylinder chamber and being slidable along an inner peripheral surface of the suck-back cylinder chamber between a first dead center position, which is a limit of movement on the side closer to the suck-back chamber, and a second dead center position, which is a limit of movement on the far side from the suck-back chamber, and a suck-back biasing member biasing the suck-back piston in a direction to increase the volume of the suck-back chamber, said suck-back cylinder chamber divided by the suck-back piston into a first fluid chamber and a second fluid chamber, said suck-back biasing member arranged in the first fluid chamber so that the suck-back piston can be moved, by supplying a drive fluid to the second fluid chamber to increase the volume of the second fluid chamber against a biasing force of the suck-back biasing member, from a suck-back position at which the suck-back piston is positioned when the increase in the volume of the suck-back chamber is completed, to a standby position at which the suck-back piston is positioned when pressure of the drive fluid in the second fluid chamber becomes the maximum, said standby position set to be away from the first dead center position.