Abstract: [Object] A diaphragm can strongly be fixed to a flow path-side body by a diaphragm-holder, a thickness of the flow path-side body of an outer periphery of a diaphragm is thinned, and the flow path-side body is downsized. [Solving Means] A diaphragm-holder abutting surface 75 is formed between a diaphragm-holder inner peripheral side ring-shaped projection 73 and a diaphragm-holder outer peripheral side ring-shaped projection 74, a diaphragm-holder inner peripheral side end surface 73s is abutted against a first fixed portion piston-side end surface 63Ap, a diaphragm-holder inner peripheral side outer peripheral surface 73o is abutted against a second fixed portion inner peripheral surface 63Bi, a diaphragm-holder outer peripheral side inner peripheral surface 74i is abutted against a body-side projection outer peripheral surface 16o, and the diaphragm-holder abutting surface 75 is abutted against the second fixed portion piston-side end surface 63Bp and a body-side projection end surface 16s.