To form a hollow double-wall structure free from deformation warpage due to cooling contraction after molding needless to say during blow molding.

The hollow double-wall structural body is composed of a front wall 2, a rear wall 3 lying opposite to the front wall 2 at distance, and a circumferential wall to form a hollow part 5 between the front wall 2 and the rear wall 3. A synthetic rod-shaped reinforcement 7 is set within the hollow part 5. The periphery of the rod-shaped reinforcement 7 is surrounded by a surrounding wall 8 integrally stretching at least to the rear wall 3. A heat insulator 10 is put between the rod-shape reinforcement 7 and the surrounding wall 8.