An image forming apparatus having a heat member (11a, 11c) for heating a recording material which bears an unfixed image thereon and recording material feeding means (8,82) configured so that a longitudinal center of the heat member becomes a conveyance center of the recording material for plural different sizes of recording material, the image forming apparatus further comprising: a central portion temperature detection part for detecting a temperature of the heat member corresponding to the conveyance center or adjacent thereto; a one side end portion temperature detection part for detecting one end portion temperature at one side in a longitudinal direction of the heat member at a position suitable for distinguishing between a large width and a small width of the recording material fed in a centered position; an other side end portion temperature detection part for detecting another end portion temperature at the other side in a longitudinal direction of the heat member at a position suitable for distinguishing between a large width and a small width of the recording material fed in a centered position; and a control part for controlling the image forming apparatus based on heat member temperature information detected by the central portion temperature detection part, the one side end portion temperature detection part, and the other side end portion temperature detection part, wherein the control part is adapted to control a power supplied from a power supply part to the heat member (11a, 11c) so that a temperature information input from the central portion temperature detection part is maintained at a predetermined set fixing temperature, and characterised in that the control part is adapted to determine that an arrangement of the recording material on the recording material feeding means (8,82) is not correctly set when one of the detection temperatures of the two end portion temperatures (TH2,TH3) is higher than the other detection temperature, and the other detection temperature is substantially the same as the detection temperature of the central portion temperature detection part, and the control part is adapted to prohibit image forming operation based on the determination. a heat member (11a, 11c) for heating a recording material which bears an unfixed image thereon and recording material feeding means (8,82) configured so that a longitudinal center of the heat member becomes a conveyance center of the recording material for plural different sizes of recording material, a central portion temperature detection part for detecting a temperature of the heat member corresponding to the conveyance center or adjacent thereto; a one side end portion temperature detection part for detecting one end portion temperature at one side in a longitudinal direction of the heat member at a position suitable for distinguishing between a large width and a small width of the recording material fed in a centered position; an other side end portion temperature detection part for detecting another end portion temperature at the other side in a longitudinal direction of the heat member at a position suitable for distinguishing between a large width and a small width of the recording material fed in a centered position; and a control part for controlling the image forming apparatus based on heat member temperature information detected by the central portion temperature detection part, the one side end portion temperature detection part, and the other side end portion temperature detection part, wherein the control part is adapted to control a power supplied from a power supply part to the heat member (11a, 11c) so that a temperature information input from the central portion temperature detection part is maintained at a predetermined set fixing temperature, and characterised in that the control part is adapted to determine that an arrangement of the recording material on the recording material feeding means (8,82) is not correctly set when one of the detection temperatures of the two end portion temperatures (TH2,TH3) is higher than the other detection temperature, and the other detection temperature is substantially the same as the detection temperature of the central portion temperature detection part, and the control part is adapted to prohibit image forming operation based on the determination.