An endless belt device includes an endless belt and a plurality of rotary members rotatably supporting the endless belt. One of the plurality of rotary members includes a marker formed on a part of a circumferential surface of a rotation axis thereof. A reflection-type sensor is arranged to oppose the circumferential surface of the rotation axis of the one of the plurality of rotary members to detect the marker formed on the circumferential surface. A rotation speed of the endless belt is detected by detecting the marker on the circumferential surface of the rotation axis of the one of the plurality of rotary members by the reflection-type sensor.