To prevent the erroneous detection of the flaw of a film over the entire width of the film by correcting the level change of a photomultiplier tube output signal caused by the change of the quantity of laser beam emitted from a laser irradiation apparatus.

The laser beam incident on a photomultiplier tube 5 is photoelectrically converted by the tube 5 to become an output signal which is, in turn, outputted to a first amplifier circuit 6 to be amplified up to a specific level to be outputted to a first integration circuit 7 and an automatic grain control circuit 10. A high voltage generation circuit 9 applies high voltage controlled by the difference voltage inputted from a first comparison circuit 8 to the photomultiplier tube 5 to control the sensitivity of the tube 5. The signal output of the circuit 16 is inputted to a second amplifier circuit 12 through a level shift circuit 11 to be outputted to a determination circuit 18. Further, the signal output of the circuit 12 of which the gain can be controlled by a control signal is inputted to a second integration circuit 14 through an absolute value circuit 13 to be integrated.