1. A freeze forecasting device for a hot water heating apparatus, for forecasting a possibility of freeze of water in a water passage circuit due to reduction in outside air temperature at a forecast target time on a forecast target day, the device comprising: a clock portion for automatically computing current year, month, day, and time; an outside air temperature sensor; an outside air temperature history storage portion for storing outside air temperatures for at least one year based on outputs from the clock portion and the outside air temperature sensor; a retrieval portion for retrieving a past outside air temperature in a time zone including a time identical to the forecast target time in a past year, month, and day corresponding to the year, month, and day of the forecast target day or a day before or after the forecast target day, from the outside air temperature history storage portion; and a forecasting portion for determining whether the retrieved past outside air temperature is not higher than a freezing critical temperature of the water in the water passage circuit and forecasting the possibility of freeze of the water in the water passage circuit at the forecast target time on the forecast target day.