A seat slide device (101, 101A) comprising: a lower rail extending in a vehicle front-rear direction and including a locked portion; an upper rail movable relative to the lower rail in a longitudinal direction of the lower rail; a lock member attached to the upper rail and including a lock portion biased in a lock direction in which the lock portion engages with the locked portion; a release lever (131, 131A) arranged at a position overlapping the lock member in a longitudinal direction of the upper rail; an operation handle (133, 133A) extending from a front side of the release lever (131, 131A) in the vehicle front-rear direction and configured to operate the release lever (131, 131A) in a lock release direction by moving integrally with the release lever (131, 131A) with a swing fulcrum portion acting as a fulcrum in response to a lock release operation, wherein the release lever (131, 131A) comprises: a release pressing portion capable of pressing a portion around the lock portion of the lock member to release an engagement of the lock portion with the locked portion; an operation portion located in the front side of the release lever (131, 131A) and configured to be operated by the lock release operation of the operation handle (133, 133A); and the swing fulcrum portion provided between the release pressing portion and the operation portion, and the lock member is a plate spring integrally comprising: a base including the swing fulcrum portion and fixed to the upper rail; a rear biasing portion located behind the base in the vehicle front-rear direction and configured to bias the lock portion in the lock direction; and a front biasing portion located in front of the base in the vehicle front-rear direction and configured to bias the operation handle (133, 133A) upward, the front biasing portion having a biasing force smaller than a biasing force of the rear biasing portion. a lower rail extending in a vehicle front-rear direction and including a locked portion; an upper rail movable relative to the lower rail in a longitudinal direction of the lower rail; a lock member attached to the upper rail and including a lock portion biased in a lock direction in which the lock portion engages with the locked portion; a release lever (131, 131A) arranged at a position overlapping the lock member in a longitudinal direction of the upper rail; an operation handle (133, 133A) extending from a front side of the release lever (131, 131A) in the vehicle front-rear direction and configured to operate the release lever (131, 131A) in a lock release direction by moving integrally with the release lever (131, 131A) with a swing fulcrum portion acting as a fulcrum in response to a lock release operation, wherein the release lever (131, 131A) comprises: a release pressing portion capable of pressing a portion around the lock portion of the lock member to release an engagement of the lock portion with the locked portion; an operation portion located in the front side of the release lever (131, 131A) and configured to be operated by the lock release operation of the operation handle (133, 133A); and the swing fulcrum portion provided between the release pressing portion and the operation portion, and a release pressing portion capable of pressing a portion around the lock portion of the lock member to release an engagement of the lock portion with the locked portion; an operation portion located in the front side of the release lever (131, 131A) and configured to be operated by the lock release operation of the operation handle (133, 133A); and the swing fulcrum portion provided between the release pressing portion and the operation portion, and the lock member is a plate spring integrally comprising: a base including the swing fulcrum portion and fixed to the upper rail; a rear biasing portion located behind the base in the vehicle front-rear direction and configured to bias the lock portion in the lock direction; and a front biasing portion located in front of the base in the vehicle front-rear direction and configured to bias the operation handle (133, 133A) upward, the front biasing portion having a biasing force smaller than a biasing force of the rear biasing portion. a base including the swing fulcrum portion and fixed to the upper rail; a rear biasing portion located behind the base in the vehicle front-rear direction and configured to bias the lock portion in the lock direction; and a front biasing portion located in front of the base in the vehicle front-rear direction and configured to bias the operation handle (133, 133A) upward, the front biasing portion having a biasing force smaller than a biasing force of the rear biasing portion.