A vehicle seat slide device includes lower rails, upper rails, screw shafts, nut members, an electric motor and a transmission mechanism. The transmission mechanism is composed of a drive reduction gear mechanism coupled directly to an output shaft of the electric motor, a pair of left and right direction conversion gear mechanisms coupled directly to the screw shafts, and a transmission shaft arranged to pass through an output gear of the drive reduction gear mechanism to be rotatably bodily with the output gear and coupled at opposite ends thereof to respective input gears of the direction conversion gear mechanisms. A lean force bracket is further provided to be secured at opposite ends thereof to the upper rails and supports the drive reduction gear mechanism and both of the direction conversion gear mechanisms.