To provide a diaphragm valve of high-speed opening and closing type capable of being held in full open position stably at the time the valve is to be opened fully.

A ball accommodating hole extending in the perpendicularly intersecting direction is provided in a valve stem 10 in such a way as to open in the full close area of a cam groove when the valve is to be opened fully, and a ball protruding partially and a spring to energize the ball outward in the radial direction are installed in the hole, and in full opening, the ball is protruded at the cam groove end on the side with full close area and abutted. Otherwise the full open area of the can groove is inclined upward at an angle of 0.10-0.17 rad (6-10 degs.) to the orthogonal surface to the axis while the full close area is inclined downward at an angle of 0.05-0.12 rad (3-7 degs.) to the orthogonal surface to the axis.