A slide rail device for a vehicle comprising: a lower rail which extends in a forward/rearward direction and is provided with a large number of lock grooves, extending in an upward/downward direction, which are arranged in the forward/rearward direction, said lower rail being immovable relative to a floor of the vehicle; an upper rail which is supported by said lower rail to be slidable in said forward/rearward direction and provided with a supporting portion; a lock spring which is provided with a supported portion, which is supported by said supporting portion, and a locking portion which is disengageably-engageable with said lock grooves, wherein a biasing force of said lock spring biases said locking portion in a direction to engage with said lock grooves; and a lock release lever, supported by said upper rail, which is rotatable between a locked position, at which said locking portion is allowed to engage with said lock grooves, and an unlocked position, at which said lock spring presses in a direction such that said locking portion escapes from said lock grooves against said biasing force, wherein said lock spring generates an engagement-assisting biasing force in a same direction or a different direction from an engaging direction of said locking portion with said lock grooves, characterized in that said supporting portion is provided with a pair of supporting surfaces, the space therebetween becoming mutually narrower with respect to the direction of said engagement-assisting biasing force, so that when said supported portion is positioned at a predetermined position by said engagement-assisting biasing force, said supported portion engages with both of said pair of supporting surfaces, so that forward/rearward movement of said supported portion relative to said supporting surfaces is restricted. a lower rail which extends in a forward/rearward direction and is provided with a large number of lock grooves, extending in an upward/downward direction, which are arranged in the forward/rearward direction, said lower rail being immovable relative to a floor of the vehicle; an upper rail which is supported by said lower rail to be slidable in said forward/rearward direction and provided with a supporting portion; a lock spring which is provided with a supported portion, which is supported by said supporting portion, and a locking portion which is disengageably-engageable with said lock grooves, wherein a biasing force of said lock spring biases said locking portion in a direction to engage with said lock grooves; and a lock release lever, supported by said upper rail, which is rotatable between a locked position, at which said locking portion is allowed to engage with said lock grooves, and an unlocked position, at which said lock spring presses in a direction such that said locking portion escapes from said lock grooves against said biasing force, wherein said lock spring generates an engagement-assisting biasing force in a same direction or a different direction from an engaging direction of said locking portion with said lock grooves, characterized in that said supporting portion is provided with a pair of supporting surfaces, the space therebetween becoming mutually narrower with respect to the direction of said engagement-assisting biasing force, so that when said supported portion is positioned at a predetermined position by said engagement-assisting biasing force, said supported portion engages with both of said pair of supporting surfaces, so that forward/rearward movement of said supported portion relative to said supporting surfaces is restricted.