The present disclosure provides a board unit including a circuit board, a connector portion mounted on the circuit board, and a case housing the circuit board. The case includes a lower case having an upper opening; an upper cover covers the upper opening, and an opening portion is formed in a side wall of the case, through which the connector portion extends inside and outside the side wall. The upper cover includes an inner circumferential surface that forms at least a portion of the opening portion and a predetermined gap with an outer circumferential surface of the connector portion. A groove portion is formed continuously along the inner circumferential surface, is open in the inner circumferential surface and is in communication with the gap. A size of the gap and a size of the groove portion are configured to form a water film by water droplets entering the case.