To provide an ice heat storage device and its water level control method capable of properly stabilizing a water level in a heat storage tank, storing ice heat without waste, stabilizing heat storage quantity, and properly deciding an additional operation time according to a load.

This ice heat storage device has the heat storage tank 12 in which a heat exchange pipe 32 is disposed in the longitudinal direction for circulating a heat exchange medium, and a main pipe 28 for supplying and a main pipe 30 for returning disposed at an upper part of an opening portion of the heat storage tank 12 for circulating the heat exchange medium to the heat exchange pipe 32. A drainage pipe 42 having a control valve 40 is connected with the heat storage tank 12, and a water supply pipe 46 having a control valve 44 is positioned in a state that it can supply water. A water temperature sensor 48, a water level sensor 50 detecting a reference water level of the heat storage tank 12, and a control device 52 for controlling opening/closing of each of the control valves 40, 44 are disposed in the heat storage tank 12. An alignment plate 18 is positioned lower than a water level in break-up in the heat storage tank 12.

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