A device for attaching fasteners has a casing having a grip, a lever supported by the casing rotatably about its upper end and urged forward, a link arm actuated by the rotation of the lever, a piston engaged slidably by the upper end of the link arm, a driving rod slidable with the piston, a hollow needle situated at the leading end of the driving rod and having a slit and a guide formed behind the hollow needle for fitting an assembly of plastic fasteners. A guide attachment which can be changed to another is removably held below the assembly fitting guide for guiding a connecting bar in the assembly in the direction of its ejection.