An electrical storage system includes a main battery, an auxiliary battery, a bidirectional DC-DC converter and a controller. The bidirectional DC-DC converter is provided between the auxiliary battery and a power supply path from the main battery to a driving motor. The bidirectional DC-DC converter steps down an output voltage from the power supply path to the auxiliary battery, and steps up an output voltage from the auxiliary battery to the power supply path. The controller controls charging and discharging of the auxiliary battery. The controller, when an allowable output power of the main battery decreases and an electric power becomes insufficient for a required vehicle output, supplies an electric power to the power supply path by discharging the auxiliary battery by using the bidirectional DC-DC converter. The controller, when an allowable input power of the main battery decreases and a regenerated electric power generated by the driving motor is not entirely charged into the main battery, charges part of the regenerated electric power into the auxiliary battery by using the bidirectional DC-DC converter.