An adjusting device provided on a transmission belt to adjust the tension of the transmission belt. The adjusting device includes a housing and a fastener fixed on the housing, the fastener engaging the transmission belt so that there is no relative displacement between the fastener and the transmission belt. A sliding mechanism is disposed on the housing and is capable of sliding in a linear direction on the housing, the sliding mechanism engaging the transmission belt so that there is no relative displacement between the sliding mechanism and the transmission belt. An elastic device is fixed to the housing at a first end and fixed to the sliding mechanism at a second end for maintaining proper tension in the transmission belt.