a circular interior circumference and the first and second pinion shafts 205 a and 205 b are inserted complimentarily thereto further inner bottom surfaces of the first groove 305 a and the second groove 305 b are planar a lower portion of the second pinion shaft 205 b is inserted into the third groove 305 c complimentarily as shown in fig2 and a lower end surface of the second pinion shaft 205 b contacts on an inner bottom surface of the third groove 305 c particularly there is a characteristic shape of an end surface of the first pinion shaft 205 a in the present exemplary embodiment as explained hereinafter referring to fig3 a center portion in the lower end surface of the first pinion shaft 205 a contacts onto an inner bottom surface of the first groove 305 a from this configuration the first pinion shaft 205 a and the first pinion gear 210 a may be coaxially aligned further a distance between the inner bottom surface of the first groove 305 a and the lower surface of the first pinion shaft 205 a become increased toward an inner circumference of first groove 305 a from the rotation shaft 310 of the first pinion shaft 205 a radially from this configuration a first space 300 a is formed between the lower end surface of the first pinion shaft 205 a and the inner bottom surface of the first groove 305 a and thus lubricant is interposed in the first space 300 a accordingly lubrication.