A vehicle seat sliding apparatus, comprising: a first rail adapted to be fixed to one of a vehicle floor and a seat and having a pair of first vertical wall portions (11, 11) aligned side by side in a width direction, a first connection wall portion connecting base ends of the first vertical wall portions, and a pair of flanges (13, 13) projecting from tip ends of the first vertical wall portions (11, 11) inwardly in the width direction in which the flanges (13, 13) face each other and further bending toward the base ends of the first vertical wall portions (11, 11); a second rail adapted to be fixed to the other one of the vehicle floor and the seat in a relatively movable manner with respect to the first rail and having a pair of second vertical wall portions (14, 14) aligned side by side in the width direction between the flanges (13, 13), a second connection wall portion connecting base ends of the second vertical wall portions (14, 14), a pair of outward extending wall portions (16, 16) projecting outward apart from each other in the width direction from tip ends of the second vertical wall portions (14, 14) while coming closer to the first connection wall portion in a top-bottom direction, and a pair of extending wall portions (17, 17) extending in the top-bottom direction from tip ends of the outward extending wall portions (16, 16) so as to be surrounded by the first vertical wall portions (11, 11) and the flanges (13, 13); characterized in that it further comprises: a pair of balls (20a, 20a) interposed in a rollable manner between a pair of second ball guides (16a, 16a) provided to the outward extending wall portions (16, 16) in inner portions in the width direction and a pair of first ball guides (18a, 18a) provided to the first connection wall portion so as to protrude to come closer to the second vertical wall portions (14, 14) in the top-bottom direction and on inner sides than the second ball guides (16a, 16a) in the width direction; and an attaching portion disposed at a tip end of the first connection wall portion in a direction of a relative movement where the first ball guides (18a, 18a) are not formed and forming a seating surface of a fastening member that is adapted to fix the first rail to one of the vehicle floor and the seat. a first rail adapted to be fixed to one of a vehicle floor and a seat and having a pair of first vertical wall portions (11, 11) aligned side by side in a width direction, a first connection wall portion connecting base ends of the first vertical wall portions, and a pair of flanges (13, 13) projecting from tip ends of the first vertical wall portions (11, 11) inwardly in the width direction in which the flanges (13, 13) face each other and further bending toward the base ends of the first vertical wall portions (11, 11); a second rail adapted to be fixed to the other one of the vehicle floor and the seat in a relatively movable manner with respect to the first rail and having a pair of second vertical wall portions (14, 14) aligned side by side in the width direction between the flanges (13, 13), a second connection wall portion connecting base ends of the second vertical wall portions (14, 14), a pair of outward extending wall portions (16, 16) projecting outward apart from each other in the width direction from tip ends of the second vertical wall portions (14, 14) while coming closer to the first connection wall portion in a top-bottom direction, and a pair of extending wall portions (17, 17) extending in the top-bottom direction from tip ends of the outward extending wall portions (16, 16) so as to be surrounded by the first vertical wall portions (11, 11) and the flanges (13, 13); characterized in that it further comprises: a pair of balls (20a, 20a) interposed in a rollable manner between a pair of second ball guides (16a, 16a) provided to the outward extending wall portions (16, 16) in inner portions in the width direction and a pair of first ball guides (18a, 18a) provided to the first connection wall portion so as to protrude to come closer to the second vertical wall portions (14, 14) in the top-bottom direction and on inner sides than the second ball guides (16a, 16a) in the width direction; and an attaching portion disposed at a tip end of the first connection wall portion in a direction of a relative movement where the first ball guides (18a, 18a) are not formed and forming a seating surface of a fastening member that is adapted to fix the first rail to one of the vehicle floor and the seat. a pair of balls (20a, 20a) interposed in a rollable manner between a pair of second ball guides (16a, 16a) provided to the outward extending wall portions (16, 16) in inner portions in the width direction and a pair of first ball guides (18a, 18a) provided to the first connection wall portion so as to protrude to come closer to the second vertical wall portions (14, 14) in the top-bottom direction and on inner sides than the second ball guides (16a, 16a) in the width direction; and an attaching portion disposed at a tip end of the first connection wall portion in a direction of a relative movement where the first ball guides (18a, 18a) are not formed and forming a seating surface of a fastening member that is adapted to fix the first rail to one of the vehicle floor and the seat.