To provide a measuring device such as a leak tester capable of guaranteeing measurement accuracy, while reducing a user load, preventing surely a failure, and thereby reducing repair cost.

The leak tester is equipped with a measuring system 31 for measuring a differential pressure (physical quantity) between a work w and a master m by air (fluid) passing through a filter 22f, a detection means 33 for detecting not only chamber pressures on both sides of the filter 22f but also the degree of foreign matter interminglement into the measuring system 31, and self-dealing means 32, 34 for performing a corresponding operation such as information based on the detected degree of foreign matter interminglement.