PURPOSE: To enable a faced quantity of fluid to flow through even at full closing of a weir in a valve in which a diaphragm is arranged to traverse the weir, and liquid flowing over the weir is limited by providing a bypass adjustable by a bypass valve on the weir.

CONSTITUTION: A weir 26 is fitted to the intermediate part of a housing 18 having an inlet part 21 and an outlet part 19, and a diaphragm 30 to abut on or separate from the weir 26 is fitted thereto. This diaphragm 30 is held on the fixed position of a pressing member 34 by a ring-like retainer 35, the pressing member 34 is vertically moved by rotational movement of a handle 38, and hence the diaphragm 30 abuts on or separates from the weir 26. In the weir 26, a bypass chamber 40 communicating to the inlet part 21 and the outlet part 19 through an inlet slot 42 and an outlet slot 44 is formed, and the opening area of the respective slots 42, 44 are adjusted by a bypass valve 46 extending in the lengthwise direction of the weir 26.