PURPOSE: To reduce the Na cleaning work at periodical inspection together with increased detection accuracy, by decreasing the sodium (Na) attached to the probe through heating it internally.

CONSTITUTION: When the power cable 7 is connected to the non-heating electric wire 4 and the probe 1 is heated to about 200°C by power application from the power supply 24 to the heater 2, the Na attached to the probe 1 is liquefied since Na has the melting point of about 100°C, and it is dropped with the self weight. Further, the tip 1A of the probe 1 is of semisphere, then the dropout of Na liquid by the weight is quickened and the amount of Na attached to the tip of probe is reduced. Further, when the Na liquid level increases and it reaches the tip 1A of probe, the liquid level is detected with the electric circuit. Thus, by decreasing the amount of Na to the probe through the reduction of Na attachment ot the probe, since the position of the liqiud level detection point of the probe tip can more accurately be forecasted, the accuracy of detection is increased and the cleaning work of Na is also reduced.