To provide a seat sliding device of long rail type enabling application of a motor thereto without causing the constitution to be complicated etc.

The seat sliding device is structured so that its drive transmitting means 16 to transmit the power of a motor 14 to upper rails 18 (L and R) is equipped with a first flexible main motion towing member 16-1 drawn in by the regular rotation of the motor and paid out by the reverse rotation and a second flexible main motion towing member 16-2 drawn in by the reverse rotation of the motor and paid out by the regular rotation. The sliding of the seat relative to the lower rails with the power of the motor 14 is made practicable to be secured by locking and coupling the terminations of the first flexible towing member 16-1 and the second flexible towing member 16-2 with the floor side member in the same row position as the mating front and rear ends of the lower rails 20 (L and R).