PURPOSE: To obtain a constant temperature container with less parts at a low cost, by forming in box-shaped blow molds a three-layered tubular parison with the intermediate layer being a foaming agent-containing resin into a hollow box, severing the hollow box into a body and a lid, and connecting them pivotally using hinges.

CONSTITUTION: The three-layered tubular parison 4 is molded such that it has the intermediate layer 3 of a foaming agent-containing synthetic resin between an inner layer 1 and an outer layer 2 of a synthetic resin that does not contain a foaming agent, then it is blow molded in the split molds having rectangular molding surfaces therein into an enclosed box-like intermediate element. The intermediate element is severed to form a container body 6 having a bottom wall 6a, and lid body 7 having a ceiling wall 7a, and after, if desired, frame members 8, 8 are attached respectively to the severed parts of the bodies 6, 7, the lid body 7 and the container body 6 are pivotally connected by the hinges 9 or the like.