To provide a safety device of a bath heater for enhancing the empty vessel heating preventing function of a bath heater, and facilitating installation and maintenance-control.

This safety device has a safety valve 60 arranged in a fuel supply passage (a fuel supply pipe 10) for supplying fuel to the bath heater 6, and opening-closing the fuel supply passage, a combustion detecting means (a thermocouple 76) for detecting combustion of the fuel, a solenoid valve unit 58 for holding the safety valve in an opening state by power supply when this combustion detecting means detects the combustion, and an overheat detecting means (an empty vessel heating detector 20) arranged in a circulating pipe part (12 or 14) of the bath heater, and closing the safety valve by releasing the power supply to the solenoid valve unit when the detecting temperature rises to the prescribed temperature or more by detecting a temperature rise.