To take out/house a spare tire by extremely simple operations, and reduce assembling manhours by reducing parts in number.

A spare tire housing chamber 12 capable of housing a spare tire 11 in a state that it is in an upright condition, is provided, and a spare tire take-in/out port 12a capable of being opened/closed by a lid 14, is formed in the spare tire housing chamber 12. The base end of a guide board 19 is mounted onto the end part opposite to the lid 14 of a bottom board main body 18 supporting the spare tire 11, and the guide board 19 is so constituted that it can be revolved around the base end in a vertical surface, and it can be reciprocated in a specified range of the distance in the direction that the spare tire 11 is taken in/out. An opening part is formed at a position faced to the tip end of the guide board 19 out of a skirt rail 22 laid along the lower peripheral inner side of a body outer panel, and the tip end of the guide board 19 is loosely inserted into the aforesaid opening part. Locking parts 19e projected out of both side peripheries at the tip end of the guide board 19, are locked over to the skirt rail 22 in such a way that they can be parted.