A power seat slide device capable of changing not only to an electric operation but also to an manual operation, and preventing half-lock in spite of a manually operable structure. For front-rear movement, a rack attached to a lower rail and a pinion meshed with the rack are used, and a driving force of a motor is transmitted to the pinion, which enables the use in an electric mode. As no slide screw is used, while the motor is not energized, rotating the pinion causes an output shaft of the motor to rotate, so that manual adjustment becomes possible.