PURPOSE: To improve the working ratio of the resin molded article and to curtail the working time by forming a resin without assemblying inserting members to the forming mold, inserting the inserted members immediately after the parting of the mold, and fixing the members by the shrinkage deformation of the resin.

CONSTITUTION: There is provided a boss part in which an insert member is installed. In this boss part there is formed an insertion port, and a cut-out portion extending from the end surface of the edge of the insertion port toward a position where the insert member is provided. Immediately after the mold release of a resin molded article, for instance, an air cleaner case A comprises a case main body a, and a cover b, both members a and b being made of a thermoplastic resin, for example, an ABS resin. In order to provide a nut 1 in the case main body a, a boss part a1 is formed. On the end surface of the edge of the insertion port 6 there is provided a cut-out portion passing through the center from the outer surface of the boss part a1.