PURPOSE: To correct automatically the purity deterioration due to external magnetic change by detecting the change in the external magnetic field with a magnetic sensor and generating a correcting magnetic field with a magnetic generator in a display with the output of the sensor.

CONSTITUTION: A coil 3 is provided around the outer circumference of a CRT fluorescent screen in matching with the tube axis of a CRT2 provided in a display 1 and a coil 4 is provided near the rear prt of the neck. The direction and amount of magnetism are detected at a sensor 5, and after the output is amplified, it is applied to the correction coil 4. The magnetic force of lines generated from the coil 4 generates a correcting magnetic field corresponding to the horizontal component of the earth magnetism at all times.