To precisely and inexpensively detect inclination of a printed medium, and avoid printing out of the printed medium, by providing an arithmetic device which stores carrying distance between the printed medium, which is carried by a carrying mechanism, and a detector and calculates inclination of the printed medium.

An printed medium 3 is carried in the auxiliary scanning direction by a printed medium carrying mechanism. First, position (XA, YA) derived from carrying distance YA shown when a detector 1 detects the edge of the printed medium which is carried by Y1 in the auxiliary scanning direction at a main scanning position A is stored in a storage. Next, the printed medium is carried by -Y1, and the detector is moved to a main-scanning directed position B. The printed medium is carried again until the detector detects the edge of the printed medium, its position (XB, YB) derived from its carrying distance YB is stored. Inclination of the printed medium in relation to the main-scanning direction is calculated using the arithmetic device based on these two positional information (XA, YA), (XB, YB).