A heat radiating type level sensor measurement of liquid level capable of preventing the errors in a case of sudden environmental temperature change, and accounting for the agitated motion of the liquid surface. The measured liquid level can be obtained as the steady state output voltage plus a difference between the initial output voltage for a present measurement and the initial output voltage for an immediately preceding measurement, with the difference multiplied by a predetermined constant. The measured liquid level for a present measurement can also be obtained as an arithmetic average of the steady state output voltage and a predetermined number of previous steady state output voltages within a prescribed range obtained by previous measurements, in a case the steady state output voltage derived at the deriving step is within the prescribed range, and as the measured liquid level obtained for an immediately preceding measurement, in a case the steady state output voltage derived at the deriving step is outside of the prescribed range.