To discriminate a focus state of a cathode ray tube accurately.

A prescribed image 14 is displayed at a prescribed position on a tube screen of a cathode ray tube and when a distance from a reference position till a prescribed position in an image is detected, a focus voltage of the cathode ray tube is changed, and when a change state of the detected distance is displayed on a prescribed display means 11a, an optical low pass filter is arranged to an optical system such as an incident section of a sensor 15. Furthermore, based on an output from the sensor 15, an under state or an over state from the just focus state is discriminated, a diameter of a beam spot is calculated based on the detected line width, and a focus adaptive value is obtained based on the discrimination of the under state or the over state and the calculated diameter of the beam spot.