PROBLEM TO BE SOLVED: To provide means to prevent an increase in the temperature of a circuit part even when power of a heating part increases in an underwater heater in which the heating part and the circuit part are integrally formed.SOLUTION: An underwater heater includes: a heating part 1 having a ceramic core member in which a heating wire is wound around the outer periphery, and a thermocouple is inserted into the inside, a metal sheath-like outer cylinder for coating the core member, and a powder-type filler subjected to swaging processing, which is put between the sheath-like outer cylinder and the core member; a circuit part 2 which includes a power supply circuit for supplying power to the heating wire, a control circuit for preventing overheating of the heating wire, a sleeve composed of metal circular pipe, whose both ends are water-tightly sealed after storing the control circuit inside, and a powder-type filler put in a space between the inner wall of the sleeve and a circuit board; and a connec
