A connector assembly including a first connector (11, 11A, 31, 31A, 41, 51, 71, 91) and a second connector (21, 61, 81, 81A) that are fitted to each other along a fitting direction such that a second contact (23, 83, 85) of the second connector is electrically connected to a first contact (13, 33, 35, 43, 73, 93) of the first connector, wherein the first connector includes a first exposed surface facing the second connector and a retention projection (13A, 33A, 43A, 73A, 92B) projecting in the fitting direction from the first exposed surface toward the second connector, the second connector includes a second exposed surface facing the first connector and a projection accommodating portion (23D, 83D, 84C) of recess shape which is formed at the second exposed surface and in which at least a part of the retention projection is accommodated, the first connector includes: a first contacting portion disposed on a lateral surface of the retention projection; a second contacting portion disposed on a lateral surface of the retention projection opposite to the lateral surface having the first contacting portion thereon, at a position closer to a base portion of the retention projection than the first contacting portion is; and a third contacting portion disposed on the first exposed surface at a position away from the lateral surface, having the first contacting portion thereon, of the retention projection in an opposite direction from the second contacting portion, and when the first connector and the second connector are fitted to each other, at least a part of the retention projection is accommodated in the projection accommodating portion, the first contacting portion and the second contacting portion separately make contact with an inner surface of the projection accommodating portion, and the third contacting portion makes contact with the second exposed surface, whereby a fitting state between the first connector and the second connector is maintained, wherein the retention projection (13A, 33A, 43A, 73A, 92B) includes a hook portion (13D, 33D, 43D, 73D, 92G) protruding in a direction perpendicular to the fitting direction and having the first contacting portion at its end, wherein the projection accommodating portion (23D, 83D) includes a receiving portion (23E, 83E) that receives the hook portion, and characterized in that a ratio LZ/LX of a distance LZ between the first contacting portion and the second contacting portion in the fitting direction to a distance LX between the first contacting portion and the third contacting portion in a direction perpendicular to the fitting direction is not greater than an incline S of a normal line with respect to the direction perpendicular to the fitting direction, the normal line being perpendicular to a tangent plane formed by the hook portion (13D, 33D, 43D, 73D, 92G) and the receiving portion (23E, 83E) contacting each other when the hook portion is received by the receiving portion. wherein the first connector includes a first exposed surface facing the second connector and a retention projection (13A, 33A, 43A, 73A, 92B) projecting in the fitting direction from the first exposed surface toward the second connector, the second connector includes a second exposed surface facing the first connector and a projection accommodating portion (23D, 83D, 84C) of recess shape which is formed at the second exposed surface and in which at least a part of the retention projection is accommodated, the first connector includes: a first contacting portion disposed on a lateral surface of the retention projection; a second contacting portion disposed on a lateral surface of the retention projection opposite to the lateral surface having the first contacting portion thereon, at a position closer to a base portion of the retention projection than the first contacting portion is; and a third contacting portion disposed on the first exposed surface at a position away from the lateral surface, having the first contacting portion thereon, of the retention projection in an opposite direction from the second contacting portion, and when the first connector and the second connector are fitted to each other, at least a part of the retention projection is accommodated in the projection accommodating portion, the first contacting portion and the second contacting portion separately make contact with an inner surface of the projection accommodating portion, and the third contacting portion makes contact with the second exposed surface, whereby a fitting state between the first connector and the second connector is maintained, wherein the retention projection (13A, 33A, 43A, 73A, 92B) includes a hook portion (13D, 33D, 43D, 73D, 92G) protruding in a direction perpendicular to the fitting direction and having the first contacting portion at its end, wherein the projection accommodating portion (23D, 83D) includes a receiving portion (23E, 83E) that receives the hook portion, and characterized in that a ratio LZ/LX of a distance LZ between the first contacting portion and the second contacting portion in the fitting direction to a distance LX between the first contacting portion and the third contacting portion in a direction perpendicular to the fitting direction is not greater than an incline S of a normal line with respect to the direction perpendicular to the fitting direction, the normal line being perpendicular to a tangent plane formed by the hook portion (13D, 33D, 43D, 73D, 92G) and the receiving portion (23E, 83E) contacting each other when the hook portion is received by the receiving portion.