A seat slide device that can change sliding resistance when a seat is made to slide in accordance with a situation is provided. A seat slide device includes a lower rail fixed to a floor of a vehicle and an upper rail fixed to a seat of the vehicle and supported to be movable with respect to the lower rail. On the upper rail, a sliding member in contact with the slid surface of the lower rail from above is provided and configured such that the sliding resistance borne by the sliding member when the upper rail is moved is changed by a load in a vertical direction borne by the seat.