PURPOSE: To properly adjust the image density by measuring the toner density of a developer immediately before it is conveyed to a development region at the tip regulation section of a blade.

CONSTITUTION: This toner density measuring device is constituted of a sleeve 2, a blade 7 arranged to face it, a power source 3 applying AC voltage between the sleeve 2 and blade 7, and a detecting circuit 10 and a resistor 11 arranged in parallel with the power source 3. The blade 7 is made of a magnetic material, and its tip portion faces the magnetic pole N1 of a magnet roller 4. Since the toner density of a developer M held by the sleeve 2 and conveyed to a development region 5 is measured, the actual density of the developer M fed for development can be measured, and the density of an image is properly adjusted by the toner density control based on the measured result.