An apparatus for detecting a liquid level includes a warning device for warning a low liquid level, and a regulator for controlling power supply to the warning device from a power source. The regulator comprises a switch for switching its condition in accordance with the liquid level. The ON condition of the switch indicates that the present liquid level is lower than a predetermined liquid level. The regulator further comprises a thermistor connected to the switch in series. The thermistor has a characteristic of decreasing the resistance value thereof, when a predetermined delay period of time as a function of atmospheric temperature has elapsed since the start of applying the voltage on the thermistor, synchronizing with turning the switch on. The low resistance value of the thermistor allows the power for activating the warning device to be supplied to its device, while the switch is turned on.