To improve the measuring efficiency and operating efficiency of a sensor by eliminating the slack of a cable connected to the sensor and a hose by extending/contracting a flexible pipe member, through which the cable and hose are passed along a guide.

When a float or a clean water draining mechanism descends during a measurement, a sensor 1 and a cleaning device 2 attached to the draining mechanism descend and a flexible pipe member 4 housing a cable 5 connected to the sensor 1 and a hose 6 connected to the cleaning device 2 also descend and extend downward along a guide 3. In addition, when the sensor 1 and cleaning device 2 ascend accompanying the float or clean water draining mechanism ascending, the pipe member 4 also ascends along the guide 3 and is contracted. Consequently the cable 5 and the hose 6 do not become slack, and the cable 5 and hose 6 do not become obstacles when the position of an interface 11 is measured with the sensor 1. In addition, the measuring efficiency and operating efficiency of the sensor 1 are improved, because the cable 5 and hose 6 do not become obstacles when the sensor 1 and cleaning device 2 move vertically.