A seat slide device that can adjust sliding resistance of sliding of an upper rail with respect to a lower rail in an unlock state is provided. The seat slide device is provided in a vehicle and includes a lower rail fixed to a floor of a vehicle, an upper rail fixed to a seat of the vehicle and supported to be capable of sliding with respect to the lower rail, a lock mechanism configured to switch a lock state in which movement of the upper rail is restricted and an unlock state in which the movement of the upper rail is allowed, and an adjusting mechanism configured to adjust sliding resistance of sliding of the upper rail with respect to the lower rail in the unlock state.