PURPOSE: To obtain the high reliability of the fault detection of an equipment and to detect water leakage in its early stage, by inputting the detected value of hydrogen density varying with time to a data processing circuit, and measuring the variation rate of the hydrogen density with time there and detecting the water leakage on the basis of whether the variation rate exceeds a specific value.

CONSTITUTION: Values Cm, Cm+1... of hydrogen density at intervals of time ΔT are used and time differences between reference time points Tm, T1... and measurement points CB, Cm... are made constant to regard variation widths as variation rates. Those variation widths, i.e. variation rates are calculted to check whether they exceed corresponding set values or not, and a decision is made in correspondence to the results. This arithmetic processing is carried out by a microprocessor or dedicated logical elements easily. This, the decision on the water leakage is made on the basis of the hydrogen density output variation rates, so the detection time is shortened. It is known that the density output is low and rises extremely slowly in, specially, the initial state of fault occurrence of the equipment, and this variaton is detected in its extremely early stage.