PURPOSE: To always hold a prescribed illuminance which is set in advance, by obtaining uniform irradiating distribution, always measuring the illuminance of irradiated light, executing adjustment automatically so that the illuminance quantity of an irradiated surface goes to a prescribed value, and recording the illuminance as time passes.

CONSTITUTION: An object to be irradiated is placed between partition plates on a turntable 5, a motor 24 is driven at a regular speed by a speed switching lever, and the table 5 is rotated. The object to be irradiated on the table 5 is irradiated by a fluorescent lamp 3 through a reflecting plate and/or a transmitting plate. Also, in the center of the table 5, an illuminance measuring use photodetector 25 is provided, a signal from this photodetector is sent to a comparing circuit 28 through an illuminance measuring circuit 26, and also an illuminance is recorded in a recorder 29 as time passes. The illuminance measuring signal which has entered into the comparing circuit 28 is compared with a reference illuminance set value which is set by an illuminance setting switch 22 in advance, and if a difference is generated between the measured value and the set value, an output signal corresponding to this difference value is supplied to a voltage control device 34 through a controlling circuit 31, a voltage of the fluorescent lamp 3 is adjusted, and the adjustment is executed automatically so that the illuminance of the irradiated surface goes to a prescribed value.