With a printing device in which a printing medium is moved relatively, and which prints on the printing medium, the surface of the medium is illuminated, a plurality of illuminated regions of the medium moving in this illuminated state are imaged to acquire a plurality of images. A region to be matched having a characteristic region within a first image is set out of the images, a similar region having the highest similarity to the characteristic region is determined within a second image that is different from the first image. The estimated movement amount of the medium is calculated from the positional relation between the region to be matched and the similar region. Error information indicating error in the estimated movement amount of the medium attributable to the angle of light directed at the medium is acquired, and the estimated movement amount is corrected.