To enhance mechanical strength of a crack part by fixing the crack part by applying an adhesive to the crack part, and filling and covering the fixed crack part, a hole and/or a goove and an area to connect these to each other with a putty like adhesive in the next place.

On the reverse side of a plastic part for an automobile where a crack is caused, a hole and/or a groove are arranged on its both sides or one side by sandwiching a crack part. The groove is arranged in a straight line shape, a curved shape and an angular line shape so as to be in parallel to or to cross the crack part. Before a putty-like adhesive B is filled and laid, an adhesive A is previously applied and fixed to the crack part. After the crack part is previously fixed by the adhesive A in this way, the putty-like adhesive B is filled in and laid on the fixed crack part on the reverse side of the part, a hole and/or a groove an an area to connect these to each other, and it is hardened.