PROBLEM TO BE SOLVED: To provide the subject system to be used for fish culture or the like, intended for preventing e.g. fish death due to water temperature rise, fire outbreak due to the overheating of a heater when the sensor or heater is exposed to the air such as the case with decreased water level in a water tank and/ or the sensor's being detached off the water tank by mistake. SOLUTION: This water temperature control system A is made up of an electric source section 1, a temperature setting section 2, a sensor section 3, a display section 4, an output section 5, a control section 6, and a heater 7, and has such a scheme that the level sensor 32 of the sensor section 3 has the function to detect the electrical resistance between the heater 7 in management water and a titanium pipe 310 including a sensor 311 therein. A voltage when both the heater 7 and the sensor 311 are laid in the identical management water is stored as reference voltage in the control section 6, sensor voltage and the reference voltage are continually compared with each other in the control section 6, and even if the temperature of the management water falls off a specified temperature range, the heater 7 is ensured not to actuate unless it is detected that both the heater 7 and the sensor 311 are laid in the identical management water.
