A post processing apparatus is provided which allows control a post processing position without printing an unnecessary mark. According to the post processing apparatus (2), a controller (21) detectsa position of an edge of paper (P) and a position of an image on the paper (P) from a read image acquired by a reader (22), and controls a post processing position in a post processor (23) on the basis of the position of the edge of the paper (P) and the position of the image detected.