PURPOSE: To improve a separating property irrespective of a kind of transfer paper or an environmental condition, by measuring and calculating thickness of transfer paper and its specific resistance value after feed of the transfer paper and before transfer, and controlling and deciding various conditions of transfer and separation in accordance with those results.

CONSTITUTION: Voltage is applied between rollers 8a, 8b, and when a transfer paper passes through both the rollers, its resistance value is measured by a measuring circuit 19, the measured value of this circuit is a resistance value of the transfer paper, and is obtained as a measuring output 20. As for a measuring output showing thickness of transfer paper, and an intrinsic output of transfer paper, its timing is measured in a timing circuit 16 by a paper feed detecting means, etc. provided just behind both the rollers 8a and 8b. The outputs are sent to the integration circuit, and are divided into several stages by the A/D converting and operating circuit, and are processed. In this way, a transfer current of the transfer unit and applied voltage to the conductive belt, etc. are controlled.