1. An image forming apparatus comprising: an image carrier capable of rotation; a forming portion configured to form a first pattern and a second pattern on said image carrier, said first pattern being used for detecting displacement of an image forming position in a rotational direction of said image carrier, said second pattern being used for detecting displacement of an image forming position in a direction perpendicular to said rotational direction; a detecting portion configured to detect said first pattern and said second pattern formed on said image carrier; and a correcting portion configured to correct, based on a detection result from said detecting portion, an image forming position associated with said forming portion, wherein: said forming portion performs a first operation and thereafter performs a second operation; at least one of said first pattern and said second pattern is formed on a first area of said image carrier during said first operation, and at least the other of said first pattern and said second pattern is formed on a second area of said image carrier during said first operation, said first area corresponding to a half of said image carrier, said second area corresponding to the remaining half of said image carrier; and said first pattern is formed, during said second operation, on an area of said image carrier where said second pattern is formed during said first operation, and said second pattern is formed, during said second operation, on an area of said image carrier where said first pattern is formed during said first operation.