PURPOSE: To prevent a foamable material from leaking due to the blowing pressure, by forcibly fitting a panel surface member between holding members provided on a jig body, clamping them between a pair of hot plates under pressure, and allowing a foamable material injected in the space of the surface member to foam.

CONSTITUTION: Resilient bodies 7 and the holding members 8 are stuck along the inner sides of a pair of supports 6 secured in parallel on the upper surface of the plane jig body 5 to form the jig 9. The panel surface member 10 having a surface heating generating body 14 attached to the inner surface and upright flanges 10a on the four corners is put on the body 5 of this jig 9 with the flanges 10a forcibly fitted between the holding members 8, and after they are clamped under pressure by the pair of hot plates 11, 12 arranged on the upper surface of the surface member 10 and on the rear of the jig 9, the foamable material 13 is injected into the space of the surface member 10, and is allowed to foam.