A valve seat 20 is arranged between the supply circuit 8 and a downstream circuit 18. A valve body 21 blocks a fluid flow between the supply circuit 8 and the downstream circuit 18 by contacting the valve seat 20. The valve body 21 connects the supply circuit 8 to the downstream circuit 18 for supplying the fluid to the downstream circuit 18 by separating from the valve seat 20. A through hole 24 extends through the valve body 21 and thus defines a part of the supply circuit 8. The through hole 24 is defined in such a manner that the fluid that has flowed from the through hole 24 proceeds between the valve seat 20 and the valve body 21 and is thus supplied to the downstream circuit 18. The valve body 21 is formed in such a manner that a pressure receiving area at an upstream side of the through hole 24 is smaller than a pressure receiving area at a downstream side of the through hole 24. This reduces a pressure loss caused by the fluid passing through a backflow prevention structure that permits forward fluid flow from the supply circuit 8 to the downstream circuit 18 but prohibits backflow of the fluid.