Assuming that N1 and N2 are the respective numbers of pinion rollers (23, 63); ρ1 is the ratio between the inner diameter of a ring roller (22) and the outer diameter of a sun roller (21); and ρ2 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 N1=3, and (ρ1+1)×(ρ2+1)≧24+16×20.5, in the case where N2=3, then it holds that ρ1≧0.102×(ρ1+1)×(ρ2+1)+1.196, and ρ1≦min[0.204×(ρ1+1)×(ρ2+1)+3.123, 7+4×30.5], and in the case where N2=4, then it holds that ρ1≧(2−20.5)×(ρ1+1)×(p2+1)/4−1, and ρ1≦min[0.185×(ρ1+1)×(ρ2+1)+1.320, 7+4×30.5].