A reference time ts, which corresponds to a period of time a sheet S needs to move passed a detection position P1 where a sheet detection sensor 841 is disposed, is set in advance in accordance with the length of the sheet S. After a time t1 from the start of driving of a gate roller, as the leading edge of the sheet S transported to a nip area N which is between the heater roller 91 and a pressurizing roller 92 arrives at the detection position P1, an output from the sheet detection sensor 841 changes to an L-level. When the transportation of the sheet has been normal, the sensor output returns to an H-level after a reference time ts or a longer time. On the contrary, upon occurrence of a jam J2 that the sheet S has got wrapped around the heater roller 91, the leading edge of the sheet S moves backward, and therefore, a duration tm in which the sensor output is kept at the L-level becomes shorter than the reference time ts.