PURPOSE: To easily determine a desired filament electric current by measuring and storing a tube voltage and current and a filament voltage and current by a trial exposure and then reading out the filament current while presetting the tube voltage and current.

CONSTITUTION: When a maximum filament current of an X-ray tube 16 is set up by a setting device 14 and in concurrence with an X-ray exposure a filament current is switched to a minimum value, a tube voltage, a tube current, a filament voltage, and a filament current output are determined respectively by measuring instruments 21∼24 and are stored in memories 34∼36 through A/D converters 25∼28. Furthermore, comparing data of an RAM33 where a relation between normal stationary filament resistance and current is stored, with those stored in the memories, characteristic data are obtained and stored in an RAM11. Therefore, when using, it can be achieved to determine a proper filament current is a short period of time by reading out the filament current for preset tube voltage and tube current.