toward said printing drum 37 is inserted into an opening in a master hold release means and grapsed by a clamp 39 provided in said drum 37 in response to a detection signal indicating that the leading end of master is correctly clamped by said clamp 39 the drum 37 starts to rotate in the direction of arrow of full line and the master is therefore wound along the periphery of said drum 37 the master is preferably so sized as that said opening on the drum 37 is covered by the trailing end of the master and such overlapping arrangement of the leading and trailing ends of master prevents the intrusion of developer into the drum 37 through said opening at the development step in a developing unit 42 and also the direct electrostatic charging on the periphery of said drum 37 it is also possible after the master is wound on the printing drum 37 to detect the charge potential of the exposed zone and of unexposed zone in a standard area previously determined on the master by means of a potential detecting means ph4 and to control the condition of electrostatic charging and or development in response to the result of said detection thereby achieving the electrostatic printing in optimum state in such case it becomes possible to dispense with the detection of master adequacy by means of said detecting means 36 stated differently optimum electrostatic printing conditions can be determined either by the signal from said automatic master adequacy detecting means 36 or that from said potential detecting means ph4 naturally the most precise determination of electrostatic printing.