PURPOSE: To accurately detect the position of an ice presence section by providing the inner wall of a heat storage tank with a plural number of detecting pipes each of which is furnished with a heater and a temperature sensor while the detecting pipes are disposed along the direction of the depth at specified intervals on the tank inner wall, and thereby comparing the output of the respective temperature sensors with one another with the action of each heater controlled.
CONSTITUTION: The inner wall of an ice heat storage tank 1 is provided with a plural number of detecting pipes 4 along the direction of the depth while the pipes are being normal to the wall, and each open end section of the pipes is projected in the inside of the tank 1. Heaters 5 are provided for the inside of each detecting pipe 4, and temperature sensors 6 are concurrently provided below each heater 5. In this case, in order to prevent each detecting pipe being thermally affected by the heaters 5 of the adjacent detecting pipes, the detecting pipes 4 are arranged in a zigzag shape in the depth direction of the ice heat storage tank 1. A temperature management device 7 directs a commands for the ON/OFF action of each heater 5, and concurrently receives temperature information detected by the respective temperature sensors 6, so that detected pieces of information is compared with one another while each level is detected. And when the inside of each detecting pipe is heated with each heater energized, the level of an ice presence section is accurately determined by enlarging and comparing the difference in temperature between latent heat at the ice presence section and an increase in temperature at an ice absence section.