Provided is a connector assembly wherein a slider has been adopted and connector-to-connector looseness has been suppressed. In this invention, a first connector 100 comprises a housing 10 and a slider 20. The slider 20 is held in the housing 10 so as to be slidable in the width direction indicated by arrows W1 and W2. Then, by sliding in the direction of the arrow W1, the slider 20 causes the second connector 200 to move in the direction of arrow F. In addition, by sliding in the direction of the arrow W2, the slider 20 causes the second connector 200 to move in the direction of arrow R. On an inner wall of the slider 20, a first protrusion 25, projecting inward, is provided. In addition, on an external wall of the second connector 200, a second protrusion 53, projecting outward, is provided. At completion of mating, the first protrusion 25 and the second protrusion 53 overlap in the up-down direction, pressing and widening the slider 20 in the up-down direction. In this manner, the external wall surface of the slider 20 is pressed against the inner wall surface of the housing 10.