A mask generating unit generates a show-through area mask in accordance with inverted front printed image data, obtained by inverting front printed image data in a horizontal direction, and back printed image data. Then, a determining unit inspects a show-through area of the back inspecting image data corresponding to the show-through area mask with a show-through inspection threshold. Consequently, an area where the show-through exists with high possibility is inspected with the looser show-through inspection threshold. This achieves suppression of determination error due to the show-through. On the other hand, an area where no show-through exists with high possibility is inspected with a tight normal inspection threshold. This achieves accurate determination of printing error. As a result, accurate printing inspection is obtainable even to web paper with the show-through.