PURPOSE: To enhance the reliability of the titled apparatus, by a method wherein the variation of a hydrogen concn. signal is detected and the comparison of a varying amount is carried out over the length of time undergoing said variation to certainly detect minute leakage in its early stages.

CONSTITUTION: A substractor 42a takes the deviation of a present hydrogen concn. signal 31 and a past hydrogen concn. signal corresponding to the primary delay value Δt of a primary delay circuit 41a while a substracter 42b takes the deviation of the past hydrogen concn. signal (Δt delay) corresponding to the primary delay value of the primary delay circuit 41a and the past hydrogen concn. signal (2Δt delay) corresponding to the primary delay value of a primary delay circuit 41b with respect to the hydrogen concn. signal. A comparator 43a carries out the comparison of the output of the substracter 42a and a set value and, if the output reaches the set value or more, trip output 51a is issued while a comparator 43b carries out the comparison of the output of the substracter 42b and a set value and, if said output reaches the set value or more, trip output 51b is issued. A logic circuit 52 carries out the decision of water leakage when both trip outputs 51a, 51b are simultaneously realized to issue a water leakage signal 53.