PURPOSE: To enhance the heat exchanging efficiency by inhibiting a biased flow in a thermal medium chamber by disposing ribs in the thermal medium chamber.

CONSTITUTION: A thermal medium chamber 3 in which a thermal medium flows is formed between a tank 1 and the inner surface of a jacket enclosing the tank. In such a hot water reservoir, the heat is exchanged between the inside of the tank 1 and the thermal medium chamber 3 through the outer wall of the tank 1. Ribs 4 bridging the outer surface of the tank 1 and the inner surface of the jacket 4 are formed in the thermal medium chamber 3. By forming an adequately long flow passage by the ribs 4, the thermal medium is allowed to flow all along the thermal medium chamber 3 without stagnant spots, and so the heat exchange between the tank 1 and the thermal medium chamber 3 can be done adequately without a waste.