Abstract: A cap member to cover a cylindrical portion of a governor case of an automatic transmission is shaped to have a cylindrical wall as its open end portion with an inner diameter larger than the outer diameter of the cylindrical portion of the governor case, and a plurality of circumferentially arranged arcuate slits are formed at intervals in the cylindrical wall of the cap. The cap put on the governor case is easily and quickly fastened by means of a resilient fastener wire which is curved into a generally arcuate shape and bent locally so as to form a plurality of V-shaped inward projections at intervals corresponding to the intervals between the slits of the cap. The fastening is completed by fitting the fastener wire around the cylindrical wall of the cap and inserting the inward projections respectively into the slits of the cap. Preferably an annular flange is formed in the cylindrical portion of the governor case as a retainer for the fastener wire.