An adjustable length adapter incorporates a driving motor on the inside and is coupled with a rotation tool at one side, wherein a rotation shaft protrudes from the rotation tool and rotates when the driving motor is driven. The adapter includes a coupling member formed with a through-hole at one side to be penetrated by the rotation shaft, wherein the rotation tool is coupled at one side of the rotation tool, a main body having a supporting member formed at the other side of the coupling member, a varying section having one side coupled with the supporting member and withdrawable therefrom, and the other side is formed with a rotation shaft to be coupled with an exchangeable working member, a driving pulley disposed on top of the main body and coupled with the rotation shaft, a driven pulley coupled with the rotation shaft of the varying section, and a belt coupled between the driving pulley and the driven pulley for rotating the driven pulley when the rotation shaft protruded from the rotation tool rotates. The main body of the adapter coupled at a side of the rotation tool is coupled with a varying section, which is coupled with a working member such as a cutting blade or the like, in such a manner that the length of the varying section is adjustable so that the length is adjusted according to the state of an object, thereby enabling easy machining of the object.