To provide a probe in a water-level gauge using a measuring tape which is new and can be used in the safe state that it will not malfunction at measurement in a narrow place.

An insulator part made of a plastic or the like is connected to the tip end of one of electrode parts made of a conductive metal tubing, the other of the electrode parts with the outer diameter smaller than that of the tip end face and the height lower than that is provided at the center of the tip end face of this insulator part, one of polar conductors extended to the one electrode part through a measuring tape from a water-level gauge is made connectable, and the other of the polar conductors is also made connectable.