Abstract: An underwater heater for warming water in an aquarium comprises a thermal sensor for sensing temperature of a main heater body and a control unit for controlling operation of the main heater body via reception of a detect signal output from the thermal sensor. In addition to a switch device for opening and closing a power-supply circuit for supplying power to the main heater body, the control unit further comprises a drive circuit, an abnormality-detection circuit, and a memory circuit. Whenever the abnormality-detection circuit detects that the thermal sensor has detected an abnormal temperature, the drive circuit activates the switch device to open itself to cause power supply to the main heater body to be shut off and to simultaneously cause the memory circuit to store the abnormality-detected condition therein.