A seat sliding device comprising: a lower rail including a slide locking engagement portion on at least one of longitudinal-direction sidewall portions; an upper rail slidably provided on the lower rail and supporting a seat; a slide locking mechanism which includes an engagement portion for slide engageable with the slide locking engagement portion and which locks the upper rail to the lower rail at an appropriate slide adjustment position; and a walk-in mechanism which reclines a seat back of the seat forward and moves the seat forward, wherein the walk-in mechanism includes a memory member which stays at the slide adjustment position independently of the upper rail at the time of a walk-in operation to control a return position of the upper rail after the walk-in operation to the slide adjustment position, wherein a portion with which the memory member is engaged in order to stay at the slide adjustment position is the slide locking engagement portion, wherein the memory member includes a memory engagement portion provided at a position corresponding to the slide locking engagement portion of the lower rail and biased in such a direction as to be engaged with the slide locking engagement portion, and wherein, on the upper rail, a memory member control mechanism is provided which, except at the time of the walk-in operation, releases the engagement of the memory engagement portion with the slide locking engagement portion to enable the memory member to follow the upper rail, and at the time of the walk-in operation, allows the memory engagement portion to be engaged with the slide locking engagement portion, characterized in that the memory member includes: a base portion supported to be movable along the sidewall portion of the lower rail; and an engagement plate portion provided integrally with the base portion, including the memory engagement portion, and having spring force biasing the memory engagement portion in the direction such that the memory engagement portion is engaged with the slide locking engagement portion. a lower rail including a slide locking engagement portion on at least one of longitudinal-direction sidewall portions; an upper rail slidably provided on the lower rail and supporting a seat; a slide locking mechanism which includes an engagement portion for slide engageable with the slide locking engagement portion and which locks the upper rail to the lower rail at an appropriate slide adjustment position; and a walk-in mechanism which reclines a seat back of the seat forward and moves the seat forward, wherein the walk-in mechanism includes a memory member which stays at the slide adjustment position independently of the upper rail at the time of a walk-in operation to control a return position of the upper rail after the walk-in operation to the slide adjustment position, wherein a portion with which the memory member is engaged in order to stay at the slide adjustment position is the slide locking engagement portion, wherein the memory member includes a memory engagement portion provided at a position corresponding to the slide locking engagement portion of the lower rail and biased in such a direction as to be engaged with the slide locking engagement portion, and wherein, on the upper rail, a memory member control mechanism is provided which, except at the time of the walk-in operation, releases the engagement of the memory engagement portion with the slide locking engagement portion to enable the memory member to follow the upper rail, and at the time of the walk-in operation, allows the memory engagement portion to be engaged with the slide locking engagement portion, characterized in that the memory member includes: a base portion supported to be movable along the sidewall portion of the lower rail; and an engagement plate portion provided integrally with the base portion, including the memory engagement portion, and having spring force biasing the memory engagement portion in the direction such that the memory engagement portion is engaged with the slide locking engagement portion. a base portion supported to be movable along the sidewall portion of the lower rail; and an engagement plate portion provided integrally with the base portion, including the memory engagement portion, and having spring force biasing the memory engagement portion in the direction such that the memory engagement portion is engaged with the slide locking engagement portion.