Image forming apparatus includes image forming units, an intermediate transfer member, a mark detecting mechanism, a counter, and a controller. The intermediate transfer member have marks. A distance between nth and (n+1)th marks is substantially equal to or smaller than a difference between a distance from a write position of the nth image forming unit to a primary transfer position of the (n+1)th image forming unit and a distance from a write position of the (n+1)th image forming unit to the primary transfer position of the (n+1)th image forming unit. The controller controls the nth image forming unit to start writing when the mark detecting mechanism detects the nth mark and the counter reaches a predetermined count value, and controls the (n+1)th image forming unit to start writing when the mark detecting mechanism detects the (n+1)th mark and the counter reaches another predetermined count value.