PURPOSE: To reduce pressure loss at the time of circulating coolant by providing a square-shaped base end in which an opening shape of a tank is the same lateral width of a joint, i.e., the same lateral width as its diameter, at the joint.

CONSTITUTION: An opening 43 of a tank 2 of a base end 41 is formed in a square shape having substantially the same lateral width as that of a joint 4 in such a manner that the area of a passage is larger than that of the joint 4. The inner surface shape of the end 41 is sequentially varied to a circular shape from the rectangular opening 43 toward the joint 4 side, and continued smoothly to the inner periphery 4a of a cylindrical shape of the joint 4. Accordingly, the inner surface 2a of the tank 4 is smoothly continued to the inner periphery 4a of the joint 4 through the inner surface of the end 41, and the area of the passage is continuously varied. Therefore, a vortex due to peeling of a boundary layer scarcely occurs in the flow of the coolant, and hence pressure loss can be reduced.