PURPOSE: To maintain the specified potential of a sheet for plate making to be electrostatically charged by electrostatic chargers connected to a power supply by measuring the surface potential of the carried sheet for plate making directly at plural points, comparing the average values thereof with a target value and controlling the power supply.

CONSTITUTION: Potential sensors S1, S2, S3 are disposed to the position where the sheet P for plate making is ejected from the chargers t1, t2. The measured potentials are inputted to a potential average value computing circuit F. The average value of the potentials of the three sensors S1, S2, S3 is calculated in said circuit. The calculated average value is sent via a high-voltage power supply control circuit G to the high-voltage power supply H to control the high-voltage power supply H. The sheet P is charged in the chargers t1, t2 by the voltage and current from the controlled high-voltage power supply H and, therefore, the always the specified surface potential of the sheet is maintained. The potential of the sheet for plate making is so controlled that the specified potential is maintained regardless of the difference in the sensitivity of the sheets for plate making and the distances between the chargers and the sheet for plate making under carrying in such a manner.