PURPOSE: To mix developer uniformly and to prolong service life by creating a relative velocity between the toner and carriers in a channel for the developer containing the unmixed toner and carriers, and mixing both.

CONSTITUTION: In the channel for the developer containing the unmixed toner 12 and carriers 13, both are mixed after creating the relative velocity between them. An electrostatic printer is provided with a highly effective hole 11 for restraining the movement of lightweight microtoner 12 in the flowing direction, by utilizing the fact that they have significantly different surface energies because of differences in specific gravity and grain diameter; by passing powder drain, composed of the carries 13 and toner 12, through the hole 11, the relative velocity is created between them, and both are mixed. Thus, the toner can be uniformly dispersed in the carriers in a short time, and stabilized for a long time.