PURPOSE: To detect defects in a test specimen by providing a mask having a transparent part and an opaque part between the test specimen and an illumi nation light source before the test specimen is irradiated with illumination light from one side to pick up its transmitted light by the light source by means of a camera from the other side.

CONSTITUTION: A test specimen 1 is transported in a predetermined direction by a transporter. Light is emitted to the test specimen 1 from an illumination light source 4 in synchronism with the image pickup signal of a TV camera 2. Defects of the specimen 1 are detected from the image pickup signal of the camera 2 by means of a defect detection circuit 3. A mask 11 having a transparent part 11a and an opaque part 11b is provided between the specimen 1 and the light source 4. This causes the specimen 1 to have a bright part 9 by the transparent part 11a of the mask 11 and a dark part 10 by its opaque part 11b. Since a bubble 8 in the specimen 1 is irradiated with no light from the dark part 10, this comes in shade to allow the outline of the bubble 8 to be detected as a contrast image. The opaque part 11b of the mask 11 is provided in a stripe or mesh form.