6 m s in fig6 a and 6b flow speeds of the airflows are represented as lengths of illustrated lines fig7 a and 7b are schematic diagrams for explaining the measurement results shown in fig6 a and 6b respectively in fig7 a and 7b the airflows flow in directions pointed to by illustrated arrows and the speeds thereof are represented by the lengths of the arrows as shown in fig6 a through 7b if the developing unit casing 7 has the concave part 12 oriented toward the magnetic brush 11 the outward airflows from the interior of the developing unit casing 7 have quite lower flow speeds than those in the conventional developing unit casing without the concave part 12 according to the measurement results it can be observed that the developing apparatus 1 has the structure that the airflows reduce the problem of splashing of toner and carrier because of the lower flow speeds if the space between the developing unit casing 7 and the magnetic brush 11 is made narrower the airflows are also forced to flow through a narrower flow path as a result the flow speeds increase and the faster airflows exit from the developing unit casing 7 in a state where momentums thereof are maintained for this reason the developing apparatus 1 can effectively prevent splashing of toner and carrier compared to the conventional developing apparatus having no concave part here although the head speed of the magnetic brush 11 varies depending on the amount of supplied developer the above mentioned tendency of the airflows persists fig8 is an enlarged view schematically illustrating the concave part 12 with respect to a depth thereof wherein the length gh represents the distance between the head of the magnetic brush 11 formed by the permanent.