A shut-off valve comprising: a valve body part that moves along an axis (A) inside a valve chamber , and has a valve body surface formed on a plane orthogonal to the axis (A); a valve seat part provided around an inlet that allows fluid to flow in the valve chamber along the axis (A), 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 (A) 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 that project from the valve seat surface toward the valve body surface along the axis (A), 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 (A) 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 (A), 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 (A) inside a valve chamber , and has a valve body surface formed on a plane orthogonal to the axis (A); a valve seat part provided around an inlet that allows fluid to flow in the valve chamber along the axis (A), 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 (A) 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 that project from the valve seat surface toward the valve body surface along the axis (A), 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 (A) 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 (A), 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 . <EOS>