A method for attaching a gear driving part of this disclosure includes a first step of inserting a positioning bolt (23) into a positioning hole (21) of a first gear from a front side, and then screwing the positioning bolt into a screw hole (24) of a fixation-side member to regulate a phase of the first gear with respect to the fixation-side member; and a second step of inserting a portion of the bolt protruding to a front side of the first gear into a hole (22) of the gear case, and meshing the first gear with the second gear while positioning the fixation-side member in a predetermined phase position of the gear case.