supplied to the input of the second absolute value generator circuit 163 is selected so that it is held at the maximum voltage level at the upper end and lower end of the display area of the screen 9 or the liquid crystal cell 7 more specifically the first and second absolute value generator circuits 162 and 163 each include a gain adjustment variable resistor vr 4 that determines the gain correcting amount and a variable resistor vr 5 that determines a balance to be adjusted the variable resistors vr 4 vr 5 are not shown in fig1 but will be described later on with reference to fig1 accordingly by adjusting the x axis and y axis gain and balance adjustment variable resistors vr 4 and vr 5 a relation between the voltage level and the screen position can be freely adjusted such that a waveform 165 shown by one dot chain line in fig1 and whose voltage level is zero at the center position of the screen 9 is changed to a waveform 166 shown by a solid line whose zero cross point 168 is moved toward the right end or toward the lower end of the screen 9 or to a waveform 167 whose zero cross point 168 is moved toward the left end or.