To detect damage to a water piping formed in the atmosphere of high pressure.

In a gas accumulated part mounted in a piping path in a drain side connected to water piping one electrode provided to protrude in an inner side in the vicinity of an upper end of this gas accumulated part, the other electrode separately provided from this one electrode to be provided to protrude in an inner side, and a conductive float connected to be wired to this other electrode to come into contact with the above one electrode in the case of fully filling the gas accumulated part with water to be lowered down with a water level lowering down according to repletion of gas to be contrarily separated from the above one electrode, are left as provided, a current value carried between the electrodes is measured, an alarm is generated when the current value is decreased less than a preset value by contrarily separating the conductive float from the electrode.