BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cushion ring that is used between a rod-side cylinder head and a piston rod of a fluid-pressure cylinder and is externally fitted on the piston rod to irushion a shock in the vicinity of the farthest extended end of cylinder, and a fluid-pressure cylinder provided with this cushion ring. 2. Description of the Related Art Of fluid-pressure cylinders using an oil pressure, a water pressure, and the like, a fluid-pressure cylinder in which a working fluid is supplied to both the bottom side and the rod side of piston to generate a linear driving force uses a cushion ring to cushion a shock produced when the working fluid is supplied to the bottom side and thereby a piston rod extends and the piston comes into contact with a rod-side cylinder head, that is, a shock in the vicinity of the farthest extended end of cylinder. One example of the fluid-pressure cylinder using such a cushion ring is explained with reference to FIG. 3. FIG. 3(a) is a longitudinal sectional view typically showing the fluid-pressure cylinder using a cushion ring that is the related art of the present invention, FIG. 8(b) is an explanatory view for explaining the operation of the cushion ring in the case where a throttling groove of the cushion ring shown in FIG. 3(a) has a V-shaped cross section, and FIG. 3(c) is a sectional view taken along the line R-R of FIG. 3(b). A fluid-pressure cylinder 30 shown in FIG. 3