A lock mechanism for a seat track slide device, the lock mechanism comprising: a lower rail configured to be fixed to a floor; an upper rail configured to be fixed to a seat and supported movably relative to the lower rail; a plurality of lock portions formed in the lower rail to be arranged in a moving direction of the upper rail; a lock member attached to the upper rail; and a handle extending from the lock member, wherein the lock member is inserted in the upper rail and has a pair of arm plates arranged side by side in a direction intersecting with the moving direction of the upper rail with an interval formed in between, the lock member is supported pivotally with respect to the upper rail by a pivot support portion, in which a recess formed in one of the upper rail and at least one of the arm plates of the lock member is fitted on a protrusion formed on the other of the upper rail and the at least one of the arm plates of the lock member, the lock member and the handle are interlocked with each other by an elastic force of a spring supported between the lock member and the handle, and the lock member can be set to a locking state or an unlocking state by operating the handle to engage or disengage the lock member with or from the lock portions of the lower rail, wherein each arm plate of the lock member has a first stopper, the spring has a second stopper, which engages with the first stoppers, and engagement between the first and second stoppers prevents the protrusion and the recess from coming off from each other in directions opposite to directions in which the protrusion and the recess are fitted together in the pivot support portion, so that the protrusion and the recess of the pivot support portion are kept fitted together, characterized in that the spring is a torsion coil spring having a coil portion and a pair of arm portions (35, 36) extending from the coil portion, one of the arm portions is supported by the handle, and the other arm portion is provided with the second stopper. a lower rail configured to be fixed to a floor; an upper rail configured to be fixed to a seat and supported movably relative to the lower rail; a plurality of lock portions formed in the lower rail to be arranged in a moving direction of the upper rail; a lock member attached to the upper rail; and a handle extending from the lock member, wherein the lock member is inserted in the upper rail and has a pair of arm plates arranged side by side in a direction intersecting with the moving direction of the upper rail with an interval formed in between, the lock member is supported pivotally with respect to the upper rail by a pivot support portion, in which a recess formed in one of the upper rail and at least one of the arm plates of the lock member is fitted on a protrusion formed on the other of the upper rail and the at least one of the arm plates of the lock member, the lock member and the handle are interlocked with each other by an elastic force of a spring supported between the lock member and the handle, and the lock member can be set to a locking state or an unlocking state by operating the handle to engage or disengage the lock member with or from the lock portions of the lower rail, wherein each arm plate of the lock member has a first stopper, the spring has a second stopper, which engages with the first stoppers, and engagement between the first and second stoppers prevents the protrusion and the recess from coming off from each other in directions opposite to directions in which the protrusion and the recess are fitted together in the pivot support portion, so that the protrusion and the recess of the pivot support portion are kept fitted together, characterized in that the spring is a torsion coil spring having a coil portion and a pair of arm portions (35, 36) extending from the coil portion, one of the arm portions is supported by the handle, and the other arm portion is provided with the second stopper.