An image forming apparatus includes a main conveyance path 13, a main discharge roller 17, and a main stacking tray 19. The sorting discharge unit 20 includes a subsidiary conveyance path 22, a change-over mechanism 23, a subsidiary discharge roller 24 and a subsidiary stacking tray 26. The subsidiary discharge roller 24 is provided on the most downstream side of the subsidiary conveyance path 22. The subsidiary stacking tray 26 is arranged above the main stacking tray 19 and the transfer sheet P discharged from the subsidiary discharge roller 24 is stacked in the subsidiary stacking tray 26. A stacking concave portion 29 which expands a stackable area of the subsidiary stacking tray 26 is formed below the subsidiary discharge roller 24.