PURPOSE: To reduce labor required for the manufacture of the entire apparatus by providing a joint portion of one side portion of one side portion in a hot water storage tank portion and one side portion in a collector portion in state where it covers the full length of both side portions, in a flexible manner so that the hot water storage tank and the collector part relatively swing, and forming the hot water storage tank part the collector part and the joint portion into blow moldings.

CONSTITUTION: At a compression part 23 for a joint part, a plurality of cutouts 24 in the lateral width direction of the tank part are formed at the blow molding time, and at parts positioning at the compression part 23 of an outgoing water pipe 8 and a return water pipe 9, bellows 25 and 26 are formed simultaneously with the blow-molding, for making it possible to make relative swinging between he pipe part on the side of the hot water storage tank part 1 and the pipe part on the side of the collector part 2. This molding is bent at the joint portion, and the hot water storage tank part and the collector part can be vertically superimposed one upon another.