To provide a seat slide device of a vehicle capable of easily securing a getting on and off space of an occupant to a third row seat, without receiving influence on a seat position of an assist seat, in the seat slide device of the vehicle having the third row seat where the assist seat can slide in a movable area of a second row seat.

A cable K5, an operation lever 341, a linking lever 345, a linking rod 350, a linking lever 38 and a cable K2 are arranged between a first lock releasing part of a front passenger seat 18 and a second lock releasing part of a rear seat 19, for releasing a slide lock of the front passenger seat 18 by operating the first lock releasing part, by interlocking with release operation of the slide lock of the second lock releasing part, when the front passenger seat 18 is positioned in the movable area of the rear seat 19, and the front passenger seat 18 and the rear seat 19 are locked to disable sliding.