A heat storage tank, comprises an inner cylinder 81 having a storage section 811 for storing liquid, an opening 812 at a lower position of the storage section 811 and a body 84 in which a liquid inflow channel 841 and a liquid outflow channel 842 are formed. The body blocks the opening 812. When the inner diameter of the inner cylinder 81 is referred to as a tank inner diameter D and the vertical length of the storage section 811 is referred to as a tank height H, D/H≦0.5 holds. Therefore, the storage section 811 is elongated in the vertical direction and the distance between the body 84, which is a main heat radiating portion, and the high temperature water region is increased and, as a result, the high temperature water region is extended in the vertical direction.