To apply the insertion method without damaging the advantages of the heat insulation and economy of the foamed polystyrene formed body by splitting the foamed polystyrene formed body into a plurality of parts, moving the parts parallel to each other, and adapting the variable width type heat insulation material to the intervals of a fitting place.

The foamed polystyrene formed body comprises a body 02 and two slide members 03. The side of the body 02 is a trapezoidal plate formed body, the slide members 03 having an inclined surface opposite to the trapezoidal inclined surface is closely attached to the trapezoidal inclined surface, both inclined surfaces are formed the inclined slide surfaces 10, and the whole width can be regulated through the parallel movement in the width direction along the inclined slide surfaces 10. The variable width type heat insulation material can regulate the width through the slide to meet the width of a fitting place, and because the thickness required for keeping the heat insulation is kept by appropriately selecting the angle of inclination of the inclined slide surfaces 10, the heat insulation can not be reduced, and the heat insulation effect can be kept for a long time.