A screw-tightening tool includes a housing, a spindle, a motor, and a power-transmitting mechanism including a sun member, a ring member, a carrier member and a planetary roller. The power-transmitting mechanism is configured such that, when the ring member moves rearward to be closer to the sun member in response to rearward movement of the spindle, the planetary roller transmits power of the motor to the spindle owing to frictional force between the planetary roller and each of a first tapered surface of the sun member and a second tapered surface of the ring member. The sun member is configured to move from a first position to a second position, which is forward of the first position, when the frictional force reaches a threshold, and to move from the second position to the first position when the frictional force falls below the threshold.