A seat track mechanism for installing beneath a vehicle seat in order to move the vehicle seat in the forward and backward directions includes a monotrack mechanism, to which a monopost structure and a single rail structure are applied. The seat track mechanism includes a structure for hook-engaging lower and upper rails constituting the mono track mechanism by hook guides. The single rail structure applied to the seat track mechanism is capable of not only reducing actuating force required for operation, but also preventing creation of excessive clearance upon stopping.