An image forming apparatus including a main carriage to move and scan in a main scanning direction, a sub-carriage connectable to the main carriage to move and scan in the main scanning direction together with the main carriage, a positioning guide provided on one of the main carriage and the sub-carriage, and a position detector provided on the other one of the main carriage and the sub-carriage. The position detector reads the positioning guide as the main carriage and the sub-carriage connect to each other to detect relative positions of the main carriage and the sub-carriage.