PURPOSE: To prevent a developer from being napped on a detection coil by arranging an auxiliary conveyance pole with a different magnetic polarity between two magnetic poles, i.e. a separating pole and the 1st development pre-conveyance pole on an included angle side.

CONSTITUTION: The included angle θ between the separating pole 12 and the 1st development pre-conveyance pole 8 is set to 110°≤θ≤130° and an attracting pole 18 and the auxiliary conveyance pole 19 are provided dividing the angle almost equally. Therefore, a magnetic body iron core 17 in a toner mixing ratio detector is never magnetized by lines of magnetic force. Thus, no developer 3 is napped on the detection coil 16. Further, the developer at this part is returned smoothly, so a measurement of a toner mixing ratio is taken accurately.