To provide a heat-insulating bag which is inexpensive and offers superior hot-insulating performance without narrowing a storage space or inviting an weight increase.

The heat-insulating bag includes an inner bag 1 and an outer bag 2 in which the inner bag 1 is inserted. An air layer is formed between both bags to form the heat-insulating bag as a double-structured heat-insulating bag. The inner bag 1 is formed into a cylindrical shape formed of two wide side portions facing almost parallel with each other and two narrow side portions which are formed integrally and adjacent to the wide side portions at both their sides and face almost parallel with each other. The inner bag 1 has a bottom and a folding opening. The outer bag 2 is formed into a cylindrical shape formed of two wide side portions facing almost parallel with each other and two narrow side portions which are formed integrally and adjacent to the wide side portions at both their sides and face almost parallel with each other. The outer bag 2 has a bottom and folding opening ends 2d, from one of which a lid portion 2f extends in a tongue shape. The folding openings of the inner and outer bags 1 and 2 are folded, in which state the heat-insulating bag is sealed up with the lid portion 2f.