To prevent the occurrence of a defect by using a metal pipe having the diameter of its end on the front end side in the inserting direction reduced relatively, and to improve working efficiency by making the end of the pipe an inclined surface, reducing the diameter of the opening area of the end by 80% or more, and having it inserted along the inclined surfaces of recessed parts.

This is the method for reinforcing a plastic hollow double wall structure 1 with a front wall 2 and a back wall 3 opposed with the clearance therebetween, and a peripheral wall 4 closing the peripheral part of the front wall 2 and the back wall 3. In the back wall 3, a plurality of recessed parts 8a, 8b expanded inside a hollow part is provided. From an insertion port 9 formed in the peripheral wall 4, a reinforcing member 6 made of the metal pipe in a rectangular column shape having the diameter of its end on the front end side in the inserting direction reduced relatively is inserted inside the hollow part. The reinforcing member 6 is fixed by making its side surface contacted with the recessed parts 8a, 8b.