A shut-off valve comprising: a valve body part that moves along an axis inside a valve chamber, and has a valve body surface formed on a plane orthogonal to the axis; a valve seat part provided around an inlet that allows fluid to flow in the valve chamber along the axis, and having a valve seat surface located at a position opposite to the valve body surface; and a moving mechanism that is connected to the valve body part, and moves the valve body part in a direction of the axis to switch to either a shut-off state where the valve body surface comes into contact with the valve seat surface, or a circulation state where the valve body surface separates from the valve seat surface, wherein the valve seat part has a plurality of endless projections (52, 53) that project from the valve seat surface toward the valve body surface along the axis, and are formed so as to surround the inlet, the valve body part has a diaphragm forming the valve body surface, an elastic member disposed on a back surface of the diaphragm, and an intermediate member connected to a piston part of the moving mechanism, the intermediate member is provided with an expanded diameter part, and a reduced diameter part whose radial width orthogonal to the axis is narrower than that of the expanded diameter part, in this order starting from a side of a tip of the intermediate member in the direction of the axis, the diaphragm has a locking part that is in a state in contact with the reduced diameter part in a state where the elastic member is in contact with the valve body surface and the tip of the intermediate member, and a periphery of the locking part is surrounded by a tip of the piston part. a valve body part that moves along an axis inside a valve chamber, and has a valve body surface formed on a plane orthogonal to the axis; a valve seat part provided around an inlet that allows fluid to flow in the valve chamber along the axis, and having a valve seat surface located at a position opposite to the valve body surface; and a moving mechanism that is connected to the valve body part, and moves the valve body part in a direction of the axis to switch to either a shut-off state where the valve body surface comes into contact with the valve seat surface, or a circulation state where the valve body surface separates from the valve seat surface, wherein the valve seat part has a plurality of endless projections (52, 53) that project from the valve seat surface toward the valve body surface along the axis, and are formed so as to surround the inlet, the valve body part has a diaphragm forming the valve body surface, an elastic member disposed on a back surface of the diaphragm, and an intermediate member connected to a piston part of the moving mechanism, the intermediate member is provided with an expanded diameter part, and a reduced diameter part whose radial width orthogonal to the axis is narrower than that of the expanded diameter part, in this order starting from a side of a tip of the intermediate member in the direction of the axis, the diaphragm has a locking part that is in a state in contact with the reduced diameter part in a state where the elastic member is in contact with the valve body surface and the tip of the intermediate member, and a periphery of the locking part is surrounded by a tip of the piston part.