A heat storage tank 10 includes a heat a body valve 4 having an inflow side passage 41 and an outflow side passage 42 and fitted into an open portion 11 of the heat storage tank, a center pipe 5 communicating with the outflow side passage and extending substantially vertically inside the tank and a mixing prevention plate 6, 60 for preventing mixing of cold water flowing in and hot water stored, wherein a through-hole 63 penetrating through the center pipe and a large number of flow holes 64 are formed in the mixing prevention plate, and the prevention mixing plate has a diameter D2 smaller than a diameter D1 of the open portion 11 of the tank.