Disclosed herein is an attaching device for extension eyelashes adapted to attach false eyelashes to natural eyelashes by overlapping the false eyelashes containing an adhesive with the natural eyelashes and applying heat to the adhesive. The device includes a body defining a space therein and shaped to be held by a hand. A support is formed on an end of the body to support the false eyelashes. An inner grip is shaped to surround the support, and grips the natural eyelashes and the false eyelashes. An outer grip is shaped to surround the inner grip, further grips the natural eyelashes and the false eyelashes that are gripped by the inner grip, and applies heat to the adhesive contained in the false eyelashes. A manipulating portion is slidably coupled to an exterior of the body and moved to sequentially operate the inner grip and the outer grip.