To reduce electric power consumption for air conditioning by preventing a concrete slab roof from rising in temperature due to sunshine so as to improve the habitability and amenity of a building, and enhance typhoon resistance, corrosion resistance and fastness properties by increasing the mounting strength of a heat insulating panel.

The heat insulating panel 1 is used for the heat insulating construction of the concrete slab roof 20. The panel 1 is provided with a square core material 2, which is made of a foamed resin and has reverse L-shaped cross-section notched parts 3 and 4 at lower edges on both right and left sides, and a skin material 10, which has approximately U-shaped cross-section bent wall parts 13 and 14 not only covering the top surface and right and left side surfaces of the core material 2 but also covering the surfaces of the right and left notched parts 3 and 4 and has outward extended overhanging parts 15 and 16, and wherein the width of the left overhanging part 15 is made larger in dimension than the depth of a recess 18 in the right bent wall part 14. The overhanging part 15 of the right panel 1 is inserted into the recess 18 in the wall part 14 of the left panel 1 adjacent to the right panel 1 and superposed on a lower wall 14a of the wall part 14 and the overhanging part 16, and the overhanging parts 15 and 16 put into the state of superposition are fixed to a concrete wall 20a by a fastening tool 21.