Assuming that N 1 and N 2 are the respective numbers of pinion rollers (23, 63); Á 1 is the ratio between the inner diameter of a ring roller (22) and the outer diameter of a sun roller (21); and Á 2 is the ratio between the inner diameter of a ring roller (62) and the outer diameter of a sun roller (61); and it holds that N 1 =3, and (Á 1 +1)×(Á 2 +1)‰¥24+16×2 0.5 , in the case where N 2 =3, then it holds that Á 1 ‰¥0.102 (Á 1 +1) × (Á 2 +1)+1.196, and Á 1 ‰¤min[0.204×(Á 1 +1)×(Á 2 +1)+3.123,7+4×3 0.5 ], and in the case where N 2 =4, then it holds that Á 1 ‰¥(2-2 0.5 ) ×(Á 1 +1) × (Á 2 +1) /4-1, and Á 1 ‰¤min[0.185×(Á 1 +1)×(Á 2 +1)+1.320,7+4×3 0.5 ].