PURPOSE: To improve accuracy for making judgement about a defect by making a comparison between a reference level and a signal from a sensor for judging the abnormality of an inspection object.

CONSTITUTION: A comparison means 41 is provided for making a comparison between a reference level from a level setting section 15 and a signal from a sensor 8. In addition, a judgement means 42 is provided for making judgement about the abnormality of an inspection object A on the basis of a signal from a comparison means 41. Light from the inspection object A is received by a sensor 8 and output from the sensor 8 is sent to the comparison means 41 and the level setting section 15. This section 15 sets a detected signal from the sensor 8 as a reference level, and a detected signal from the sensor 8 after detecting a group of patterns separated from the reference level is compared with the reference level by the comparison means 41. The result of the comparison through the means 41 is sent to a judgment means 42. According to this construction, a defect can be accurately detected, using two defect judgement signals detected at dephased cycles, through the repetition of the foregoing process.