the output of ff in fig1 2 there are and gates g11 g14 of which the gate g13 serves to generate cpy signal which operates a relay k for executing the printing cycle the high voltage source hvt1 hvt2 etc is switched on by the relay k and the delay signal representing the lapse of the time u03c4 2 to be described after the operation of the switches sw1 and sw2 and q signals from ff1 and ff2 are applied as input to set the flip flop ff3 the flip flop ff3 is reset after completion of a predetermined number of prints or by a stop signal although not shown the control circuit 119 is reset into a stand by condition by the print start signal str an example of the voltage control effected by a microcomputer will now be described in connection with the circuit of fig1 and the flow chart of fig1 when power is supplied to the printer and the automatic voltage setting control section 119 the program for flow execution stored in the memory starts to be executed and the limits of the master density are read into the memory 212 from the digital switches set1 and set2 the contents of the memory are revised to the latest set values by set1 and set2 until signal str is supplied from the panel 120 subsequently the signal str is read into the memory and when the master is moved signals ph a and ph b are generated these signals are read into cpu 208 through an.