To provide a broken piece detecting sensor assembly capable of stably detecting broken piece admixed in a fluid, which broken piece is made of material characterized by non-metal, non-magnetic and non-electroconductive characteristics such as ceramics. This broken piece detecting sensor assembly is a sensor for detecting broken piece admixed in a fluid. Provided are two opposed flat plates, a shift mechanism for moving at least one of the two flat plates in a confronting direction to allow the broken piece to be sandwiched between those two flat plates, and measuring and determining section. The measuring and determining section is operable to measure the distance between the two flat plates to thereby detect the presence or absence of the broken piece, the size of the broken piece or the amount of the broken piece accumulated.