When a predetermined rupture load is applied to a mounting bracket formed on an attachment part, a rupture develops in the mounting bracket, and the attachment part can be separated from a base part by this rupture. The mounting bracket, in which a slit extending in a direction intersecting a loading direction of a rupture load is formed, includes a rigid portion disposed at one side of the slit and a breakable portion disposed at the other side of the slit which is breakable because of the presence of the bolt.