In an electrophotographic developing device, its doctor blade disposed adjacent to a magnetic roll so as to control the thickness of a layer of magnetic developing agent conveyed by the magnetic roll has an edge which is curved in such a manner that the gap between the edge and the magnetic roll is a minimum in the vicinity of the middle of the magnetic roll, whereby the layer of magnetic developing agent is made uniform in thickness and the printing density is made uniform despite gravitational sagging of the roll.