There is provided a roller screw in which a twisted amount of an unloaded return path is reduced.The roller screw has a screw shaft 1 having a spiral roller-rolling surface is on an outer peripheral surface thereof; a nut 2 having a spiral loaded roller-rolling surface 2a on an inner peripheral surface thereof, the spiral loaded roller-rolling surface being opposed to the roller-rolling surface is of the screw shaft 1; a circulation member 7 arranged in the nut 2 and formed to have an unloaded return path 8 connecting one end and the other end of the loaded roller-rolling surface 2a, whose turn number is less than one, of the nut 2; and a plurality of rollers 4 arranged in a one-turn roller circulation path composed by a loaded roller-rolling path 6 and the unloaded return path 8. The circulation member 7 guides the rollers which have moved and arrived at the one end of the loaded roller-rolling path 6 having less than one turn, to the unloaded return path 8, and then returns the rollers to the other end of the loaded roller-rolling path 6 having less than one turn. The number of turns of the loaded roller-rolling path 6 is set to be less than one turn, whereby the rollers 4 which have arrived at one end of the loaded roller-rolling path 6 can be transferred to the other end of the loaded roller-rolling path 6 without changing their attitudes.