A copying machine includes a document skew detection section. The document skew detection section includes an angle calculation section and a judgment section. The angle calculation section specifies an angle θ1 formed between a reference direction and an edge of a document placed on a platen glass, in accordance with first input image data obtained by scanning the document the document being started to be scanned when an angle formed between the platen glass and a platen cover is γ°. Further, the angle calculation section specifies an angle θ2 formed between the reference direction and the edge of the document, in accordance with second input image data obtained by scanning the document, the document being started to be scanned when the angle formed between the platen glass and the platen cover is 0° Also, the judgment section generates an angular difference α obtained by carrying out an operation of θ1−θ2. This makes it possible that the document which is unintentionally skewed be identified.