In a patch measurement device, a data storage section stores printed-image data including a control strip on a printed material. Based on the pixel values constituting printed-image data stored in the data storage section, a patch position detection section detects the position of a patch. A color density measurement section measures the color density of the patch whose position has been detected by the patch position detection section. A correlation coefficient ρm between a key pattern x and subject data y which is calculated by a reference mark detection section is represented as ρm=([x]*[y])−([x]−1*[y]), where [x]*[y] is a sum of multiplication products of corresponding elements of the two matrices. Matrix [x]−1 represents an inverted pattern of the key pattern x. Even if the subject data y is of an unrelated pattern resembling the key pattern x having different signal levels from those of the key pattern x, the resultant correlation coefficient ρm has a small value, thereby indicative of a low correlation. The resultant correlation coefficient ρm also becomes small if the subject data y is that of a solid patch, due to cancellation by a drastic subtraction.