PURPOSE: To make it possible to indicate the position of mercury according to a signal by generating the signal corresponding to the physical existence of mercury with a dielectric proximity sensor in relation to a mercury movable in a container.

CONSTITUTION: For example, legs 96, 98 of plastic tube 93 are set in a block 91 via a pair of holes 95, 97 slidable therealong. The block 91 has therein a receptacle 92 in which a proximity sensor S is inserted, and the acting surface FS thereof is in a slidable contact with the wall of the tube 93. The legs 96, 98 are respectively connected to pressure sources. The level of mercury goes down at the leg 96 connected to a high pressure space while it goes up at the leg 98 connected to a low pressure space. When the surface of mercury in the leg 96 passes through the front of the acting surface FS of the sensor S, the sensor S generates a signal of indicating that the surface of the mercury is on the same level of the sensor S.