A slide rail device for use with a jump seat that can be jumped up and rotated to a storage area in an automobile, in which a locking mechanism is provided for locking and unlocking the jump seat with respect to an lower rail member fixed in the floor of automobile. The locking mechanism includes a female engagement member fixed at the jump seat, a lock member having a male engagement portion for engagement with that female engagement member, and an actuator arm operatively connected to the lock member. When the jump seat is unlocked from the slide rail device, the female engagement member is withdrawn in the lower rail member out of engagement with the female engagement member, so that there is no projection of male engagement portion of the locking mechanism from the lower rail member, which avoids undesired contact and damage between the locking mechanism and baggage or article loaded on the floor. The actuator arm may be contacted by the female engagement member, and pushed thereby to cause rotation of the lock member to again bring the male engagement portion into engagement with the female engagement member for locking the jump seat to the lower rail member.