PURPOSE: To know the exchange time of a sacrificial anode bar exactly and quickly by a method in which when the sacrificial anode bar is consumed, the lead wire of a temperature sensor is short-circuited to the inside of the fitting hole of the sacrificial anode bar.

CONSTITUTION: A fitting screw 12 to hold the upper end of a sacrificial anode bar 11 is screwed with a threshold hole 10. A temperature sensor-fitting hole 13 is provided in the upper end of the bar 11 in non-connected state with the internal space of a box 9, having a temperature sensor 14 inside. The sensor 14 may only contact with the upper end of the bar 11. One end of the lead wire 15 is thrust through the central hole 12a of the screw 12 and connected to the sensor 14, and an insulating cover 15a on one end side of the wire 15 is peeled off n the hole 13 to expose the conductive part 15b. When the bar is consumed during long-period use, water in the box 9 begins permeate into the hole 13 and comes to the exposed conductive part 15b of the wire 15, whereupon the part 15b is shortcircuited with the wire 15 to quickly lower signals (resistance values) to be transmitted to hot water temperature displayers 16 and 17 from the sensor 14. The exchange time of the bar 11 can thus be determined quickly and exactly.