Guiding pieces on a bracket cover is engaged with attachment claws on a bracket body. This engagement serves to hold the bracket cover on the bracket body when the bracket cover is set at a first position. When the bracket cover is set at a second position, the engagement between an engagement piece and a hook piece serves to cooperate with the engagement between the guiding pieces and the attachment claws so as to hold the bracket cover on the bracket body. The bracket cover is firmly held on the bracket body when the bracket cover is set at the second position. The bracket cover is thus reliably prevented from detachment from the bracket body.