A seat slide device includes a reinforcing member attached to a rear end part of an upper rail. The reinforcing member is higher in rigidity than the upper rail. The reinforcing member includes a reinforcing member body having a U-shaped cross section having a lower opening, and including a reinforcing member top wall; a pair of reinforcing member lateral walls; and a pair of reinforcing member bent wall parts each being bent outwardly from a lower end of a corresponding one of the reinforcing member lateral walls in a lateral direction of the reinforcing member, and extending upwardly between a corresponding lower rail outside lateral wall and a corresponding lower rail inside lateral wall.