An image forming system detects a first conveyance abnormality of a recording material when a time taken for the recording material conveyed along a conveyance path in the image forming system to pass a predetermined position exceeds a first threshold value, temporarily stops the conveyance of the recording material when the first conveyance abnormality is detected and changes the first threshold value to a second threshold value larger than the first threshold value when the conveyance of the recording material is temporarily stopped, and detects a second conveyance abnormality based on the second threshold when a re-conveyance is made to eject the recording material remaining in the conveyance path.