Defect of an acrylic plate is detected such that a light beam, which dominantly includes non-parallel rays not parallel to a principal surface of the acrylic plate, is introduced through a side surface formed substantially perpendicularly to the principal surface of the acrylic plate. In this arrangement, transmitted light, which is obtained through the principal surface of the acrylic plate on the basis of the defect, is detected by using any one of or both of a light-receiving device with its light-receiving surface arranged and directed substantially in parallel to the principal surface of the acrylic plate and a light-receiving device with its light-receiving surface arranged and directed substantially perpendicularly to the principal surface of the acrylic plate. Thus, the defect of the acrylic plate is quantitatively detected by measuring the amount of light of the transmitted light. Accordingly, an identical optical system can be used to perform the detection of, for example, the surface scratch of the acrylic plate as well as the bubble and the foreign matter in the acrylic plate simultaneously with the detection of the defect such as the bend and the waviness of the acrylic plate itself.