PURPOSE:To suppress undesired defect data by ANDing mask data read out of memories where the data are written alternately at intervals of a scan with defect data of one scan, and ORing defect data of plural scans convolutionally. CONSTITUTION:A defect detector 5 scans a transparent plate which runs lengthwise with a light spot in the breadthwise direction and detects and outputs the kind, size, position, etc., of a defect to a defect data fetching circuit 30. When detect data is fetched, an X-axial counter 11 outputs its counted value as X-coordinate position data and a Y-axial counter 14 outputs its counted value as Y-coordinate position data. Mask data are written in the 1st memory 31 and 2nd memory 32 alternately and read out alternately at intervals of a scan. An AND circuit 34 ANDs the read mask data with the fetched defect data of one scan and an OR unit 12 ORs data of plural scans outputted by the circuit 34 convolutionally and outputs the defect data to an FIFO memory 15. Consequently, the undesired defect data can be suppressed.
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