A battery pack (30) is configured to couple to a power tool (10) by sliding in a sliding direction and it contains a plurality of cells (31). A pair of battery-side, electric-power terminals (312a, 312b, 314a, 314b) and a battery-side signal terminal (313) can be electrically connected to a pair of tool-side, power-supply terminals (212, 214) and a tool-side signal terminal (213), respectively, which are provided on the power tool (10). The pair of battery-side, electric-power terminals (312a, 312b, 314a, 314b) and the battery-side signal terminal (313) are disposed in parallel adjacently in a direction that intersects the sliding direction.