A fixing device includes a detecting device including a contact portion contacting one end of an endless belt with respect to a widthwise direction, an urging portion for urging the contact portion toward the one end of the endless belt with respect to the widthwise direction, and a sensor for detecting a position of the contact portion. The detecting device detects a position of the endless belt with respect to the widthwise direction depending on an output of the sensor. A displacing device displaces one end of the roller with respect to a longitudinal direction depending on the position of the endless belt detected by the detecting device. Before rotation of the endless belt is started, the displacing device displaces the one end of the roller with respect to the longitudinal direction so that the contact portion moves against an urging force of the urging portion upon the start of rotation.