To solve problems wherein detection of a plurality of heights is difficult by a method of deforming a diaphragm with pressure by height difference between inner and outer liquid levels of a detection pipe to thereby operate a switch, and subsequently, the plurality of heights can be hardly detected corresponding to difference of alarms and to a lapse of time.

In a leakage detecting method of a mechanical seal for reserving leaking liquid from the mechanical seal 15 in reserving tubs 20 and 20A and for recognizing the amount of leaking liquid based on the varying level height, heights of a plurality of set liquid levels at positions having a small amount of leaking liquid and positions having a large amount of leaking liquid are made detectable.