PURPOSE: To provide a frequency with a value between two scheduled limit values and to provide the stability of a displayed character by providing a control means for adapting an adjustment value so as not to make a difference between an oscillation frequency and a scheduled required frequency exceed a scheduled value.

CONSTITUTION: A clock signal generator is provided with a start-stop oscillator 1 provided with the first input SS to which cyclic control signals REF are impressed and the different input to which the adjustment value F is impressed, the start-stop oscillator 1 supplies clock signals CLK and the clock signals are impressed to a counter 2. The counter 2 generates and impresses a counted value C to a control circuit 3 and the control circuit 3 is adapted so as to generate the value F adjusted corresponding to the received counted value C and a reference value R stored in the control circuit. The control circuit 3 separately receives line signals REF by,the input IRQ and impresses reset signals CLR to the counter 2. Thus, the frequency is corrected only at the time of reaching the scheduled limit value and firm and stable images are obtained.