A lock comprising: two lock cylinders, a cam located between the two lock cylinders; the two lock cylinders located corresponding two ends of the cam, each lock cylinder including a rotary part in a rotational direction thereof, each rotary part including a transmission slot defined axially therein, and a driver located in the transmission slot, each driver including a ridge extending radially therefrom, each of the lock cylinders having a resilient member which axially and resiliently contacts the driver corresponding thereto so as to resiliently push the driver toward the cam, each driver having a reception portion formed to an outer periphery thereof; the cam including a passage defined axially therein, the passage communicating with a path defined in an inner periphery of the passage, the path located corresponding to the ridge so that the ridge is movable axially in the path, the ridge driving and rotating the cam when the driver rotates, the path including a restriction unit located in a radial direction of the path, the restriction unit being resilient engaged with the reception portion when the restriction unit is aligned with the reception portion; when in an initial position, the two drivers contact directly or indirectly with each other in the passage, the restriction units are not located corresponding to the reception portions, when one of the two drivers is not biased by the resilient member corresponding thereto, the driver that is not biased by the resilient member corresponding thereto is pushed by another one of the resilient members and is pushed away from the path, and the reception portion of another one of the two drivers is engaged with the restriction unit corresponding thereto; characterized in that each of the lock cylinders includes a slot defined axially therein, a stop member is located corresponding to the slot and located between the lock cylinder and the driver, the stop member contacts and covers the driver in an axial direction of the slot. two lock cylinders, a cam located between the two lock cylinders; the two lock cylinders located corresponding two ends of the cam, each lock cylinder including a rotary part in a rotational direction thereof, each rotary part including a transmission slot defined axially therein, and a driver located in the transmission slot, each driver including a ridge extending radially therefrom, each of the lock cylinders having a resilient member which axially and resiliently contacts the driver corresponding thereto so as to resiliently push the driver toward the cam, each driver having a reception portion formed to an outer periphery thereof; the cam including a passage defined axially therein, the passage communicating with a path defined in an inner periphery of the passage, the path located corresponding to the ridge so that the ridge is movable axially in the path, the ridge driving and rotating the cam when the driver rotates, the path including a restriction unit located in a radial direction of the path, the restriction unit being resilient engaged with the reception portion when the restriction unit is aligned with the reception portion; when in an initial position, the two drivers contact directly or indirectly with each other in the passage, the restriction units are not located corresponding to the reception portions, when one of the two drivers is not biased by the resilient member corresponding thereto, the driver that is not biased by the resilient member corresponding thereto is pushed by another one of the resilient members and is pushed away from the path, and the reception portion of another one of the two drivers is engaged with the restriction unit corresponding thereto; characterized in that each of the lock cylinders includes a slot defined axially therein, a stop member is located corresponding to the slot and located between the lock cylinder and the driver, the stop member contacts and covers the driver in an axial direction of the slot.