an object of inside - quality inspection being conveyed on a conveyer is irradiated sideways with beams of light from projection lamps concentratedly . a photodetecting device detects light transmitted through the object irrespective of the size and thickness of the skin of the object without being influenced by extraneous light . a white - level calibrating device is provided to the photodetecting means . the projection lamps project and concentrate their beams of light onto one side of the object in an inspection position at different angles from different positions diagonally distributed from the oblique front to the oblique back . fractions of light transmitted through the object and emerging from the other side of the object are condensed by a condenser lens a shutter for opening / closing the optical path is provided between a light receiving window of the condenser lens and a light - inputting face of an optical fiber for directing light to a spectrometer . the shutter is operated when the inspection center of the object passes the inspection position . a white - level calibrating plate is moved to a position in front of the light receiving window and moved back so as to perform calibration . the deviation of the calibration curve is corrected by placing a quality reference material in the same position and moving it back .