There is provided 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 (21) attached to a lower rail (11) and a pinion (22) meshed with the rack (21) are used, and a driving force of a motor (51) is transmitted to the pinion (22), which enables the use in an electric mode. On the other hand, because no slide screw is used, while the motor (51) is not energized, rotating the pinion (51) causes an output shaft of the motor (51) to rotate, so that manual adjustment becomes possible.