PURPOSE: To prevent mis-control and danger on a human body by performing corona discharge by a base body current detecting charger only at the time of judging the installation of a sensitized body from the detected result of pattern density.

CONSTITUTION: A detection signal (data) corresponding to the reflected light quantity is read from a density sensor 29 and compared with the preset reference value. In the case of density data>reference value, that is, in the case of a sensitized drum 2 being installed, a flag A is set to '0'. The driving of the sensitized drum 2 is started upon setting the flag A to '0' and an electrostatic charger 14 is turned on to perform corona discharge. The surface of the sensitized drum 2 is thereby charged to the specified potential, and a current corresponding to the charged potential flows to its base body. The value of the base body current is then read through a base body current detecting device 30. Except in the case of density data>reference value, the flag A is set to '1' so as to flash the abnormality display part of a control panel 41. Mis-control and danger on a human body can be thereby prevented.