An image forming apparatus includes a plurality of loop detecting sensors which detect various levels of loops formed on a recording material; and a controlling device which selects a specific one out of the loop sensors in accordance with information on the recording material, and then, switches a conveyance speed of a fixing roller pair in response to a detection signal output from the specific sensor, so as to control in such a manner as to keep the loop of the recording material within a constant range, the controlling device selecting the loop detecting sensor for detecting a loop of a level in accordance with the information on the recording material, so as to perform a loop keeping control for the recording material in response to the detection signal output from the selected loop detecting sensor.