To facilitate production while keeping the required strength by connecting a plastic frame to a high-strength member for receiving the load applied from front and rear wheels at the time of traveling.

As materials for car body pipes 1, plastic materials having the strength equal with or higher than the strength of light metal such as aluminium or heavy metal such as iron is used by mixing this metal or any different material such as glass fiber, for example. Then, the lower parts of plastic frames 2 integrally molded for constituting a pair of left and right car body side parts are respectively attached to the car body pipes 1. At respective upper edge parts 2A in the lengthwise direction of the frames 2, two recessed parts 2B are formed at an interval. Namely, the recessed parts 2B are formed by providing loop-shaped projecting parts 2C higher than the ridge lines of upper edge pots 2A of the frames 2.