A thimble 61 formed with graduations 65 and numerals 67 on the outer circumference thereof at predetermined intervals is molded by double-color molding using two kinds of resin 62, 63 with different colors. After a molding product having the graduations 65 protruding outwardly on the outer circumference of a base tube 64 is formed of the resin 62, an outer skin 66 covering the outside of the base tube 64 except for the graduations 65 is formed of the resin 63. Thereafter, the numerals 67 is formed on the outer skin 66 by means of laser marking. Similarly, an outer sleeve is formed with graduations and numerals on the outer circumferential surface along the axial direction at predetermined intervals.