A pattern checking method wherein an image of a certain position of one pattern is detected; the detected image is positioned with respect to an image of a position corresponding to the certain position, in a reference pattern image; and the positioned images are compared with each other, whereby a discrepant place among these positioned images is judged as a defect the positioning operations of the images of the detected patterns are controlled based upon either pattern information such as density of the images of the detected patterns, or information obtained from the positioning operations for images of other positions in the patterns. An image sensor unit for detecting images of patterns is constructed which has such a structure that a plurality of one-dimensional image sensors functioning as a pattern detector are arranged in a two-dimensional form, and an output of a certain one-dimensional image sensor for imaging a certain position of one pattern is delayed for a predetermined time period, and also both of an output of an one-dimensional image sensor adjoining the first-mentioned one-dimensional image sensor, which images the same position of the same pattern, and the delayed output of the certain one-dimensional image sensor are sequentially added to derive a summation output. The image sensor unit is inclined at a predetermined angle with respect to a plane perpendicular to the reflection light from the patterns, and the reflection light from the patterns is focused via a confocal focusing optical system onto this image sensor unit.