In a lower cover 15, an end face of an upper edge portion 19 of a first cover-lateral-plate part 16 constitutes a first engagement end face having a first engagement groove 24 extending in a first direction, whereas, in an on-vehicle component, a lower edge portion of a first component-lateral-plate part constitutes a first engagement edge portion to be inserted into and engageable with the first engagement groove 24. In the lower cover 15, an end face of an upper edge portion of a second cover-lateral-plate part 17 constitutes a second engagement end face having a second engagement groove 28 extending in a second direction, whereas, in the on-vehicle component, a lower edge portion of a second component-lateral-plate part constitutes a second engagement edge portion to be inserted into and engageable with the second engagement groove 28. In the first engagement end face, a first inward projection 23 is formed extending in the first direction so as to demark another side, in a second direction, of the first engagement groove 24. The first inward projection 23 has a first inclined surface which gradually decreases a projection height from the groove bottom of the first engagement groove 24 toward the other side in the second direction.