PURPOSE: To obtain a rigid connecting structure, by a method wherein a thermoplastic second molded member, having lapped parts onto the predetermined section of a thermoplastic first molded member, are molded integrally through multi-material or multi-color molding, and recesses in the lapping direction are formed on the predetermined sections and the lapping sections, while projected parts to be inserted into the recesses, are formed on the other part of the lapping sections.

CONSTITUTION: A thermoplastic first molding member 1 is provided with recesses 3a, 3b at both ends (a), (b) thereof while a thermoplastic second molding member 2 is provided with projections 4a, 4b to be inserted into the recesses 3a, 3b. The shrinkage stresses F1, F2 of the second molding member 2 are generated in opposing directions as a whole whereby the projections 4a, 4b are connected rigidly to the insides 5a, 5b of the recesses 3a, 3b. As a result, the connecting structure is formed so as to have close connection on at least one surface of the recesses and projections, therefore, the connecting structure is provided with effect of water proofing also. Further, auxiliary materials, such as an adhesive agent, a packing or the like, and another energy such as ultrasonic or the like, are not required for bonding, therefore, working man-hours or the number of parts may be reduced and it may contribute to automation of the production.