A cathode current detecting circuit capable of detecting a cathode current without delay and without lowering frequency characteristics of an applied video signal. This cathode current detecting circuit comprises a generator for generating a cathode current detection reference signal on the basis of a synchronization signal, an adder for adding the detection reference signal to a video signal, an amplifier for amplifying the added video signal, a transistor for applying the amplified video signal to a cathode of a cathode-ray tube, a current/voltage converter resistor for converting a cathode current 8 into a voltage, a frequency characteristic compensating capacitor connected in parallel to the transistor and a switch connected in series to the capacitor, wherein the switch is controlled in synchronization with the detection reference signal so as to be turned on during an active video signal display period and turned off during the cathode current detection period.