PURPOSE: To enable an internal abnormality detecting sensor to detect an abnormal condition of a speed reducer in a housing by making a current flow to an electrode to which an abrasive powder or the like is attached.

CONSTITUTION: This detecting sensor is a sensor to detect an internal abnormality in an apparatus 10 which is furnished with a mechanical transmission 12 in housings 13 and 16. The sensor is provided with a first electrode 41 projecting from the inner wall 13a of the housing 13 and 16, and a second electrode 42 to form a specific clearance between the first electrode 41, wherein either of the first electrode 41 and the second electrode 42, or the housing side thereof is magnetized, and a specific voltage is applied between the first and the second electrodes 41 and 42.