To provide a heat insulating member and its method for manufacturing, easily molded and assembled, hardly marked by small impact shock in conveying and assembling, and further having fire resistance.

This heat insulating member 10 disposed between a hot water storage tank 12 and an outer case 13 surrounding the hot water storage tank 12, and having an inner face agreed with the outer shape of the hot water storage tank 12, and an outer face agreed with the inner shape of the outer case 13, further has an expandable resin 19, and a noncombustible sheet 22, 23 joined and disposed in a state of covering an outer surface of the expandable resin 19 excluding at least its upper and lower sections.