PURPOSE: To obtain invariably accurate toner density regardless of the upward/ downward movement of a rod type member for prevention against clogging by sampling a signal from a detection coil synchronously with the rod member.

CONSTITUTION: A toner density detection body 1 detects the toner density as variation in inductance by the detection coil when a developer in a funnel-shaped hopper part 2 flows from an orifice 3. A bracket 5 is provided at the upper part and a turning arm 7 having an insertion rod 6 is driven with a push lever 9. A relay 21 and a solenoid 13 are driven with a timing signal from a circuit 20 to drive the lever 9 against a spring 12, moving up and down the rod 6 in a hopper 2. A signal from the coil 4 and signal from the circuit 20 are inputted to a sample holding circuit 25 to extract the signal only when the rod 6 moves up, thus detecting the toner density.