Seat slide device for vehicle

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 each made of a synthetic resin, for supporting movement of the upper rail relative to the lower rail, 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;
• and
• at least two high-strength balls each made of a material having a Young's modulus higher than each of the plurality of guide balls, the high-strength balls being disposed such that the plurality of guide balls are sandwiched between said at least two high-strength balls, in an array direction on each of the plurality of ball rolling grooves of the upper rail and the ball rolling grooves of the lower rail.