One object is to increase the amount of abrasion powder accumulated in a sensing region to improve the sensitivity in sensing the abrasion powder. Provided is a sensor for sensing reduction of electric resistance between electrodes, a magnetic field being applied between the electrodes to accumulate magnetic powder floating in a lubricant between the electrodes, wherein at least one sensing regionin which the magnetic powder is to be accumulated is provided in at least a part of a region between the electrodes, and the magnetic powder is inhibited from being accumulated in a non-sensing region constituted by a space around the electrodes other than the at least one sensing region.