A seat slide device includes: a pair of seat slide rails configured to support a seat, each of the seat slide rails including a lower rail and an upper rail; a transfer shaft rotationally driven by motor drive in a state where the transfer shaft is provided in a bridging manner between the seat slide rails, the transfer shaft having a hollow portion; and rail drive mechanisms, each of the rail drive mechanisms being provided between the lower rail and the upper rail and being configured to move the upper rail relative to the lower rail based on a driving force transmitted from a motor via the transfer shaft.