A terminal box comprising: a box casing including terminal elements incorporated into the interior thereof; a lid element for closing an opening of the box casing; and an engaging mechanism for fixedly engaging an engaging element provided in the lid element with an engaged element provided in the box casing by pressing the lid element against the box casing; the engaging element including a projecting piece extending in a pressing direction of the lid member, and an engaging pawl extending in a direction vertical to the extending direction of the projecting piece and provided in part of the projecting piece other than a distal end portion of the projecting piece, the engaged element having a receiving hole opening toward an outer side of the box casing and allowing the distal end portion of the projecting piece to be exposed to the outside when engaged with the engaging pawl, the projecting piece is elastically deformable against a pressing force toward the box casing, the engaged element has a pair of vertical ribs extending along the pressing direction, and an intermediate plate-like beam element spaced apart from the outer surface of the box casing for connecting the vertical ribs at intermediate portions thereof, and characterised in that a connection rib is provided in an outer surface of the box casing for connecting the pair of vertical ribs at distal end portions thereof in the pressing direction and that the projecting piece has a bottom surface at the distal end portion thereof opposed to a top surface of the connection rib acting as a wall surface portion of an inner wall surface of the receiving hole positioned adjacent a downward side of the projecting piece in the pressing direction. a box casing including terminal elements incorporated into the interior thereof; a lid element for closing an opening of the box casing; and an engaging mechanism for fixedly engaging an engaging element provided in the lid element with an engaged element provided in the box casing by pressing the lid element against the box casing; the engaging element including a projecting piece extending in a pressing direction of the lid member, and an engaging pawl extending in a direction vertical to the extending direction of the projecting piece and provided in part of the projecting piece other than a distal end portion of the projecting piece, the engaged element having a receiving hole opening toward an outer side of the box casing and allowing the distal end portion of the projecting piece to be exposed to the outside when engaged with the engaging pawl, the projecting piece is elastically deformable against a pressing force toward the box casing, the engaged element has a pair of vertical ribs extending along the pressing direction, and an intermediate plate-like beam element spaced apart from the outer surface of the box casing for connecting the vertical ribs at intermediate portions thereof, and characterised in that a connection rib is provided in an outer surface of the box casing for connecting the pair of vertical ribs at distal end portions thereof in the pressing direction and that the projecting piece has a bottom surface at the distal end portion thereof opposed to a top surface of the connection rib acting as a wall surface portion of an inner wall surface of the receiving hole positioned adjacent a downward side of the projecting piece in the pressing direction.