1. A patch measurement device for measuring color density of a patch in a control strip printed on paper, the paper having a reference mark for facilitating positioning printed at a predetermined distance from the patch, wherein the patch measurement device comprises: a storage section for storing imaged data representing at least the control strip and the reference mark on the paper; a reference mark detection section for detecting the reference mark based on the imaged data stored in the storage section; a pixel extraction section for extracting, from the imaged data stored in the storage section, pixels composing the patch and surrounding pixels by referring to the reference mark detected by the reference mark detection section; a patch position detection section for detecting a position of the patch based on values of the pixel extracted by the pixel extraction section; and a color density measurement section for measuring color density of the patch based on the value of at least one pixel located at the position detected by the patch position detection section.