A tension device for a drive chain of a bicycle including a first pulley having a first rotational axis swingable around a swing center, and a second pulley having a second rotational axis movable along a movable direction to change a distance between the first rotational axis and the second rotational axis. The first pulley is arranged to be forced around the swing center to apply tension to the drive chain, and the second pulley is arranged to be forced along the moving direction to apply tension to the drive chain.