PURPOSE: To obtain a constant temperature device having an excellent heat exchanging efficiency and a simple manufacture by foam molding a heat insulator layer between an inner tank and a sheathing wall, and press bonding to hold a heat exchanger to an outer surface of an inner tank wall by a foam heat insulator layer.

CONSTITUTION: An inner tank 1 made of a stainless steel having a pair of upper and lower jigs 6 for mounting a heat exchanger at predetermined positions on an outer surface of a tank wall is prepared, and aluminum plate-like heat exchangers 4 are inserted and supported to the jigs 6. Surfaces of the exchangers 4 in contact with an outer surface 12 of an inner tank wall is coated, for example, with corrosion inhibiting polyester paint. Then, periphery of the exchanger corresponding to an inlet of a gap between the exchanger 4 and the surface 12 is coated, for example, with a silicone sealing material 7. A sheathing wall 3 is provided at a predetermined position at an outside of the exchanger 4. A foamable synthetic resin material such as, for example, two-component raw material of hard foam polyurethane is injected between the inner tank and the sheathing wall to obtain a heat insulator layer 2, an the exchanger 4 is strongly press bonded to the surface 12 by foaming pressure of the material to ensure its position.