A reduction gear (49) for a walking assistance system is provided that includes first to third planetary gear mechanisms (P1 to P3) between an input shaft (Si) connected to a motor (48) and an output shaft (So), the second planetary gear mechanism (P2) being disposed radially outside the first planetary gear mechanism (P1) connected to the input shaft (Si), and the third planetary gear mechanism (P3) being disposed outside, in the direction of an axis L, the first planetary gear mechanism (P1). It is therefore possible, while reducing the speed of rotation of the input shaft (Si) in three stages by the first to the third planetary gear mechanisms (P1 to P3) and transmitting the rotation to the output shaft (So), to reduce the thickness of the reduction gear (49) compared with a case in which the first to the third planetary gear mechanisms (P1 to P3) are disposed so as to be stacked in the direction of the axis (L), thereby improving the appearance when a user is fitted with the walking assistance system.