This planetary roller driving device includes: a planetary roller speed up gear having a first carrier being a first input axis and a first sun roller being a first output axis; a planetary roller reduction gear having a second sun roller being a second input axis and a second carrier being a second output axis, the second input axis being joined with the first output axis; a roller ring driving mechanism; a housing which accommodates the planetary roller speed up gear, the planetary roller reduction gear, and the roller ring driving mechanism; a roller ring rotation control circuit which controls a rotation of the roller ring driving mechanism corresponding to an external environmental condition; and a torque-assisting mechanism which is provided in the housing and provides an assisting torque to the first input axis or the second output axis.