PURPOSE: To automatically correct a fluctuation of horizontal amplitude and high voltage based on switching of a horizontal frequency, in the multiscan monitor allowed to correspond to plural horizontal frequencies.

CONSTITUTION: When a horizontal frequency is 15K, a first power source B1 is supplied to the other end F of a primary winding N1 of a fly-back transformer 1 through a third diode D3 and a horizontal output operation is executed. In this case, each first and second drive circuit 2, 4, and each first and second switch circuit 3, 5 are in a turn-off state, respectively. With respect to the horizontal frequency, when for instance, 31K is selected, a first drive circuit 2 and a first switch circuit 3 become a turn-on state, and a first power source B1 is supplied to a first intermediate tap T1. As a result, by what is called a boost-up effect, a supply power source voltage of a horizontal output transistor TR5 becomes high, therefore, high voltage and horizontal amplitude are corrected to proper values. When the horizontal frequency is 24K, a first power source B1 is supplied to a second intermediate tap T2 in the same way.