PURPOSE: To check luminance inclination with simple constitution when a fluorescent character display tube of filament AC voltage driving type is driven by DC voltage, by preparing inclination to the thickness of a filter to be arranged on the side of the display surface.

CONSTITUTION: The inclination balanced with the luminance inclination is provided to the thickness of a filter to be prepared on the side of the display surface. Thereby, when a fluorescent character display tube of filament AC voltage driving type is driven by DC volage, the luminance inclination can be checked even when potential difference is always generated between both ends of the filament 3.