A battery-side conduction connector (22) is disposed on a bottom surface of a case (12) housing battery packs (14, 16). The conduction connector (22) has one first terminal (for example, a female positive electrode terminal (66)) having a first polarity and two second terminals (for example, female negative electrode terminals (68a, 68b)) having a second polarity. The first terminal is positioned in a center part of the bottom surface, and the two second terminals are line-symmetrically disposed with respect to the first terminal.