<p>A toner density detecting device is provided with a coil (12) buried in a housing (11) and a cover member (13) provided on one end of the coil (12) to prevent the developing agent from touching the coil (12). The cover member (13) has a front end portion (A) provided near the surface of the housing (11) and extending along the developing agent flowing direction (Y) and an apex portion (D) at the back of the front end portion (A) with respect to therdirection (Y). The apex portion (D) projects deeper into the developing agent than the front end portion (A). An edge (ℓ) connecting the front end portion (A) and the apex portion (D) extends in the direction (Y) to cross the central axis of the coil (12). A pair of faces extending in the direction on either side of the edge (f) are gradually declined from the edge toward the surface of the housing (11).</p>
