A connector provided in a lower middle part of a battery pack, is protected from moisture in a reliable manner while simplifying the installing and removing of the battery pack. A vertical wall of the battery pack is formed with an arch-shaped ridge having a higher middle part and a pair of lower lateral end parts. A battery pack receiving recess is provided with a vertical wall opposing the vertical wall of the battery pack, and a pair of through holes on either lower end thereof. The vertical wall of the battery pack receiving recess is formed with an arch-shaped groove configured to receive the arch-shaped ridge of the battery pack. Moisture that may be deposited on the vertical wall surfaces is trapped by a water channel defined by the groove and the ridge, and drained from the battery pack receiving recess via the through holes.