To provide the leak valve which possesses a leak flow passage free of change in a leak flow rate even if there exist deformation caused by the swelling of a valve seat, the gouging by fluid, abrasion and the pressing of a diaphragm, and also possesses a structure excellent in fabrication properties for the leak flow passage.

In a diaphragm valve where a diaphragm 13 is pressed to a valve seat 9 so as to be closely attached by a pressing piece 16, a seat collar 8 integrally including a flange 7 at its outer circumferential lower end, is provided for the outlet opening periphery in the valve chamber bottom surface of a fluid inflow passage, the valve seat 9 higher in height than the seat collar 8 is fitted in the outer circumference of the seat collar 8 so as to be held therein, the inner circumference of a seat ring 12 provided with a fluid passing hole 11 communicated with a fluid outflow passage 3 in the circumferential direction, is coupled in the step part 10 in the outer circumference of the valve seat 9, furthermore the outer circumference of the seat ring 12 is also fitted in the inner circumference of a valve chamber 4, and a leak flow passage 17 from the fluid inflow passage 2 side to the fluid outflow passage 2 side is thereby formed in a space between the seat collar 8 and a valve chamber bottom surface or in the seat collar 8 itself.