Provided is a power seat slide device capable of switching to not only electric but also manual operation, and preventing half-lock in spite of a manually operable structure. For forward/backward movement, a rack (21) fitted to a lower rail (11) and a pinion (22) in threaded engagement with the rack are used, and driving force from a motor (51) is transmitted to the pinion (22). This configuration enables use in electric mode. Meanwhile, no slide screw is used, so that when the motor (51) is not energized, an output shaft of the motor (51) can be rotated by rotating the pinion, thus enabling manual adjustment.