An adapter for coupling a lever to continuously unidirectionally drive a strap fastener is disclosed. The adapter includes a U-shaped frame with two free ends and a junction. The two free ends are separately provided with two shaft holes. Two sliding slots are separately formed near the shaft holes. A lever hole is formed at the junction. A ratchet shaft has an outer section with a radial hole and an inner section inserted in the shaft holes. A bolt is inserted into the radial hole. A ratchet is mounted around the ratchet shaft. A pawl is slidably disposed between the two sliding slots for engaging with the ratchet. A spring stopper is fixed between two sides of the frame. A spring is compressively arranged between the pawl and spring stopper. A holed plate is fixed between the spring stopper and the junction.