An image forming apparatus includes an incorrect attachment or detachment preventing mechanism configured to prevent attachment and detachment of the image forming unit in the transmission state. The incorrect attachment and detachment preventing mechanism includes a locking member capable of moving to a restrict position where the locking member restricts movement of the image forming unit from the predetermined position and a permission position where the locking member permits movement of the image forming unit from the predetermined position. In the transmission state, the locking member is configured to engage with the first engaging part of the manual lever at the restrict position to prevent movement of the image forming unit. In the transmission release state, the locking member is configured to be moved to the permission position and then engaged with the second engaging part of the manual lever to permit movement of the image forming unit.