Abstract: A reservoir assembly comprises a pair of cup-shaped housing members each made of thermoplastic resin and having an opening end, the housing members being integrally secured by heat welding to each other at their opening ends, and a partition plate arranged within the housing members to subdivide the interior of the reservoir assembly into two chambers. In the reservoir assembly, one of the housing members is formed at the opening end portion thereof with a stepped inner wall which is coupled with the outer periphery of the partition plate in such a manner that the partition plate is positioned inside the opening end of the housing member and fastened in place by an annular bead which is radially inwardly extruded at the opening end of the housing member during the heat welding process.