PURPOSE: To easily detect the quantity of developer by detecting change in a magnetic flux caused by the rotation of a magnet roll by a detecting means when the quantity of the developer is reduced in a storing means.

CONSTITUTION: A detecting coil 22 is installed near a developing sleeve 16, and the opening part of the detecting coil 22 is opposite to the developing sleeve 16. A detection circuit 40 is connected with the detecting coil 22. In such a case, the developer sticking on the developing sleeve 16 is made to be in a napped state by a magnetic force by the magnet roll 18 and the rolling of the napping is repeated by rotating the magnet roll 18, then, the magnetic toner 12 is triboelectrostatically charged with a negative polarity by mutual friction between toner and toner, or the carrier 14 and the surface of the developing sleeve 16, etc. Thus, when the magnetic toner 12 is consumed and reduced, the toner quantity in the hopper 11 in the developing device is detected by the detecting coil 22, and then, the warning is given.