To eliminate an ambient temperature correcting sensor and further to improve the sensor response speed in a heat radiating type liquid level sensing system, a constant current pulse is periodically passed through a level sensor to heat it; level sensor voltages sampled at predetermined time intervals when the level sensor is being heated are divided by each initial sensor voltage to obtain sensor voltage ratios with respect to time; an approximate voltage is calculated on the basis of the the obtained ratios in accordance with an approximation equation; a liquid level is calculated and displayed on the basis of the calculated approximate sensor voltage. Further, it is preferable that the system is provided with an additional inclination correcting sensor or an acceleration sensor, in order to correct the inclined liquid level.