In the US, we use a split-phase power system: two wires (call 'em A and B) each carry AC voltage a half-cycle out of phase with each other, and a third wire functions as ground. Need 110 volts? Attach your appliance to wires A and ground, or to wires B and ground. Need 220 volts? Attach your appliance to wires A and B.
Is Japan’s electrical system for residential buildings set up this way? Or can you only get 100 volts in a house?