A rotation-transmitting mechanism includes a first planetary gear mechanism and a locking mechanism. The first planetary gear mechanism transmits rotational driving force to the transportation unit through a first planet gear to rotate the transportation roller when the motor rotates in a first direction, and transmits rotational driving force to the transportation unit through a second planet gear to rotate the transportation roller when the motor rotates in a second direction. The locking mechanism can lock a planetary rocking member of the first planetary gear mechanism in such a condition that rotational driving force of the motor is transmitted to the transportation unit through the second planet gear. The locking mechanism terminates locking of the planetary rocking member of the first planetary gear mechanism by engaging with the sheet in the transportation path downstream of the transportation roller with respect to the direction in which the sheet is transported.