PURPOSE: To prevent the crush of a metal panel generating when a bending work is conducted by a method wherein a thin plate, which is formed in corrugated shape, is used as the spacing material of a radiation panel.

CONSTITUTION: A radiation panel is formed by arranging corrugated spacer materials 5 between two sheets of metal panels 1. As the length L1 of the crest of the corrugated spacer material 5 can be made small, the strut parts, to be used to prevent collapse against the bending when the area of a flow passage 3 per unit length L of the spacer material 5 is formed equal to the conventional structure, can be formed finely in a plurality of numbers. To be more precise, as a plurality of fine strut parts can be formed, the supporting force resisting to crashing force of the corrugated constitution increases. Also, as the length L2 of the plane part when internal pressure is made shorter, the swelling of the upper and the lower metal panels 1 when the internal pressure is received is reduced, and as a result, the plate thickness of the upper and the lower metal panels 1 can be made thinner, the application of excessive force when the bending work is conducted is unneccessitated, and the degree of crushing can also be reduced.