In a main flow passage, a first heat exchanger, a first heat storage unit, a second heat exchanger, and a second heat storage unit are connected by a heating medium flow passage. The main flow passage allows a heating medium to be circulated. A sub flow passage includes a shortened flow passage which is a part of the heating medium flow passage and branches from the heating medium flow passage between the second heat exchanger and the second heat storage unit and extends to the first heat storage unit. The sub flow passage allows circulation of the heating medium between the first heat storage unit and the second heat exchanger. A first heating means in a middle of the shortened flow passage, the first heating means heating a passing heat medium, and a switching means conducting switching between the main flow passage and the sub flow passage are provided.