A seat slide device for a vehicle, comprising: an upper rail made of an aluminum alloy, the upper rail comprising: a mounting section to be fixed to a seat; and a sliding section having a plurality of ball rolling grooves each formed along a longitudinal direction of the upper rail; a lower rail made of an aluminum alloy, the lower rail comprising: a fixing section to be fixed to a vehicle floor; a guide groove for receiving the sliding section of the upper rail; and ball rolling grooves each formed in the guide groove along a longitudinal direction of the lower rail to be opposed to the plurality of ball rolling grooves of the sliding section; a plurality of guide balls for supporting movement of the upper rail relative to the lower rail, characterized in that each of the plurality guide balls is made of a synthetic resin; the plurality of guide balls being arrayed in line without using a cage in a portion between each of the plurality of ball rolling grooves of the upper rail and each of the ball rolling grooves of the lower rail, which are opposed to each other, the seat slide device further comprising high-strength balls each made of a material having a Young's modulus higher than the each of the large number of guide balls, the high-strength balls being arranged on both ends of the plurality of guide balls in an array direction on the each of the plurality of ball rolling grooves (23,33) of the upper rail and the ball rolling grooves (23,33) of the lower rail, wherein a Young's modulus of the synthetic resin forming each of the large number of guide balls is lower than a Young's modulus of each of the upper rail and the lower rail, wherein the high-strength balls are sandwiched by stoppers (24,35) which are provided at both the upper rail and the lower rail to restrict a longitudinal movement between the upper rail and lower rail, wherein a diameter of each of the high-strength balls is larger than a diameter of the each of the plurality of guide balls. an upper rail made of an aluminum alloy, the upper rail comprising: a mounting section to be fixed to a seat; and a sliding section having a plurality of ball rolling grooves each formed along a longitudinal direction of the upper rail; a mounting section to be fixed to a seat; and a sliding section having a plurality of ball rolling grooves each formed along a longitudinal direction of the upper rail; a lower rail made of an aluminum alloy, the lower rail comprising: a fixing section to be fixed to a vehicle floor; a guide groove for receiving the sliding section of the upper rail; and ball rolling grooves each formed in the guide groove along a longitudinal direction of the lower rail to be opposed to the plurality of ball rolling grooves of the sliding section; a fixing section to be fixed to a vehicle floor; a guide groove for receiving the sliding section of the upper rail; and ball rolling grooves each formed in the guide groove along a longitudinal direction of the lower rail to be opposed to the plurality of ball rolling grooves of the sliding section; a plurality of guide balls for supporting movement of the upper rail relative to the lower rail, characterized in that each of the plurality guide balls is made of a synthetic resin; the plurality of guide balls being arrayed in line without using a cage in a portion between each of the plurality of ball rolling grooves of the upper rail and each of the ball rolling grooves of the lower rail, which are opposed to each other, the seat slide device further comprising high-strength balls each made of a material having a Young's modulus higher than the each of the large number of guide balls, the high-strength balls being arranged on both ends of the plurality of guide balls in an array direction on the each of the plurality of ball rolling grooves (23,33) of the upper rail and the ball rolling grooves (23,33) of the lower rail, wherein a Young's modulus of the synthetic resin forming each of the large number of guide balls is lower than a Young's modulus of each of the upper rail and the lower rail, wherein the high-strength balls are sandwiched by stoppers (24,35) which are provided at both the upper rail and the lower rail to restrict a longitudinal movement between the upper rail and lower rail, wherein a diameter of each of the high-strength balls is larger than a diameter of the each of the plurality of guide balls.