PURPOSE: To improve serviceability at low cost by reducing components in number while preventing X-ray radiation, by turning a thyristor ON on detecting an abnormal high voltage developing and then by disusing a component which is easy to get out of order.

CONSTITUTION: In case that a high-tension output across winding 3b of fly-back transformer 3 increases abnormally, a pulse voltage across protective circuit supply winding 3d falls in the negative direction to decrease the gate voltage of thyristor 15 below its anode voltage, and consequently thyristor 15 conducts to stop horizontal oscillation circuit 1 from stopping. Next while the collector voltage of transistor Tr7 is normal, a voltage at the connection point between resistances 23 and 24 is set larger than the sum of the forward voltage of diode 25 and the collector of Tr7 is disconnected from the connection point between resistances 23 and 24. Once a short circuit occurs to load circuit 6 or diode 4, the collector voltage of Tr7 falls to make diode 25 conductive, and the gate voltage of thyristor 15 also falls below a fixed value, so that thyristor 15 will conducts to stop circuit 1 from operating.