During a first operation, at least one of a first pattern and a second pattern is formed on a first area of an image carrier, and at least the other pattern is formed on a second area of the image carrier. The first area corresponds to a half of the image carrier, while the second area corresponds to the remaining half of the image carrier. During a second operation, a second pattern is formed on an area of the image carrier where a first pattern is formed during the first operation, and a first pattern is formed on an area of the image carrier where a second pattern is formed during the first operation.