In a position deviation detecting method for an image forming device, first and second patterns formed in different colors on image supporting mediums are transferred to an intermediate transfer medium in an overlapping manner, the first patterns formed in a color with high sensor sensitivity and the second patterns formed in a color with low sensor sensitivity. The first and second patterns on the transfer medium are read using a light-receiving unit of a pattern detection sensor. An amount of position deviation in a main scanning direction between two of a plurality of colors is detected. The first patterns are set in a fixed position in the main scanning direction and the second patterns are shifted. Output values of the light-receiving unit are measured from the first and second patterns, and the amount of position deviation in the main scanning direction is computed based the measured output values.