PURPOSE: To prevent defective sepn., by changing the electric current of a sepn. charger in accordance with the detection signal for the light transmissivity of transfer paper.

CONSTITUTION: The comparison of transferring and separating characteristics between plain paper and transparent paper indicates that, if the AC component of separating current is increased with the plain paper, the range (a) of the separable DC component increases but the range (b) of a good image decreases. Therefore, the condition under which said paper is separated without spoiling image quality is about 125μA AC component and about 23μA DC component. In the case of the transparent paper, if the AC component is ≤50W125μA, the condition under which the paper is separated is not found no matter how the DC component is changed, and the paper is separated only when the AC component is increased to ≥150μA. Thus, the common condition for sepn. in the range where the image is good does not present for the plain paper and the tracing paper. Thereupon, a light emitting lamp 16 is provided between conveyance rolls 16 for the transfer paper, and a photo IC 17 is placed on the opposite side of transfer paper 5 to detect the quantity of the light transmitted through said paper. Said IC applies a signal to an AC power source to change the DC component and AC component of the current for sepn. according to the transmissivity of the transfer paper. The corona sepn. with good image quality is thus accomplished in conformity with the kind of the transfer paper.