An adjustable wrench includes a handle having a head portion provided with a fixed jaw, a transverse guiding slot and an opening communicated with the guiding slot. A movable jaw has a guiding portion received movably in the guiding slot of the handle and a rack on the guiding portion. An adjusting worm has a through hole and a helical tooth, wherein the adjusting worm is received in the opening and engages the helical tooth with the rack of the movable jaw. A supporting device is made of a flexible material and has at least a part received in the through hole of the adjusting worm to make the adjusting worm can be turned along the supporting device and can be moved along a diameter orientation thereof, and a driving device is provided on the handle for moving the adjusting worm toward the movable jaw.