A fixing device is mounted on a main casing of an image forming apparatus. A heating member contacts a recording medium to apply heat thereto while rotating. A heating member includes a roller that rotates about an axis. A frame supports the heating member and includes a frame fixing part that is fixed to the main casing to mount the frame on the main casing. A detecting unit opposes the heating member in an opposing direction to detect a temperature of the heating member and is separated from the heating member. A support member supports the detecting unit. A position of the support member in the opposing direction with respect to the frame is determined at a position closer to the frame fixing unit than the detecting unit. A position of the support member in an axial direction of the roller with respect to the frame is determined at a same position as the detecting unit in the axial direction.