To provide a backflow preventive unit assembly equipped with two check valve assemblies.

A backflow preventive unit assembly 10 has a housing 20, a first fitting assembly 43: constituted so as to be insertedly fitted into the first end part 22 of the housing, and a second fitting assembly 53: constituted so as to be insertedly fitted into the second end part 24 of the housing. The housing and the fitting assemblies have through bores 30, 48, and 58 fluid-communicated with each other. Fixing materials 34 and 38 fix the opposite surface of the first fitting assembly and the housing, and the opposite surface of the second- fitting assembly and the housing respectively. A plating material is provided in the inner surfaces of the housing and the fitting assembly, and forms a wall for limiting the movement of the fixing materials into the housing and the bore of the fitting assembly. A ball valve and a check valve are fitted to the fitting assembly, and the wall of the housing is equipped with a port for approaching the check valve.