To make it possible to easily vary the width of a plate width variable thermal insulating material and to provide the plate width variable thermal insulating material capable of applying an insertion method without damaging merit of thermal insulating efficiency and economical efficiency, etc., of a molding body.

A plate-like body having proper plate thickness molded from an independent bubble foam of synthetic resin such as foam polystyrene molding body, etc., width and length is so constituted that it is divided in the direction of the width with side edges 11 and inclined (an angle of inclination θ) linear divided slide edges 4 (41 and 42) seeing from the plate surface. Contact positions of the divided slide edges 41 and 42 are sliped lengthwise (arrow X) of the plate-like body to adjust the plate width W.