An adapter is placed between a power outlet and a power plug to electrically connect them. The adapter includes a pair of blade slots and a pair of terminal slots arranged in correspondence with a pair of blades and a pair of earth terminals protruding from a power plug. The pair of blade slots is arranged on a first reference line passing through a center of an adapter body in a radial direction, and is arranged laterally symmetrical with respect to a second reference line. The pair of terminal slots is arranged on the second reference line, and is arranged vertically symmetrical with respect to the first reference line.