A chassis is formed of a one-piece thermoplastic or thermosetting resin body. A bottom member extends in a bottom plane, and has at least three side members extending in respective planes. Junction portions join the bottom member and the side members. Ribs are intermittently molded as one piece to bottom member and to side member and extend from the bottom plane from the bottom member through the junction portion to the side plane of the side member. An opening portion helps to prevent angular deformation of the resin body between the bottom member and the side member from a desired angle between the bottom plane and the side plane when the one-piece body is molded by minimizing differences in the cooling rate at different parts adjacent to the junction portion in the one-piece body. The opening portion is located between the ribs at least adjacent to and along the junction portion. The opening portion has a length along the junction portion that is at least five/ten the length of the junction portion.