Abstract: This invention relates to a discriminating type flaw detector for detecting the types, sizes and locations of flaws existing in a light-transmitting plate material with high accuracy and high speed. A light-spot scanner (11) repeatedly scans a light-transmitting plate material with a light spot in the direction normal to the travelling direction of the light-transmitting plate material. When the light spot falls on a flaw in the light-transmitting plate material, the conditions of transmitted light, transmitted and diffused light, reflected light and reflected and diffused light change. These changes are detected by light receptors (12), converted into electrical signals in photoelectric converters (13), and processed into flaw data in a flaw data generating circuit (14). A flaw data acquisition circuit (15) collects flaw data, prepares a flaw pattern by combining the flaw data into flaw data blocks by flaw, and reads the addresses of the flaw data blocks as location data.