To prevent the compression quantity of a seal member from being restricted due to its fitting position between cases, and to provide satisfactory seal property according to an increase in a water pressure by increasing a face pressure in proportion to the water pressure.

A seal member 20 is formed between a first case 16A and a second case 16B forming a casing. The seal member 20 is provided with a butted section butted to a first face 20c including a fixed section fixed to the internal face of the first case 16A and a second case 16B shaped like a convex at the opposite face of the first face. A concave section 16Aa2 is closed with the first face 20c outside the fixed section of the seal member 20, and connected to the outside of the first case 16A by a passage 16Aa3.