Three battery cells can be arranged such that one battery cell is situated on the front side of a handle portion and that the remaining two battery cells are situated on the rear side thereof while being arranged laterally side by side. Also, a handle portion can be generally of a triangular cross-sectional configuration whose apex portion is situated on the front side of the handle portion and whose base portion is situated on the rear side thereof; and the handle portion is grasped by bringing fingertips into contact with the apex portion (11a) on the front side.