A cogeneration system comprising: a combined heat and power unit configured to generate electricity and heat; a heat accumulator configured to store a heat medium having recovered the heat generated by the combined heat and power unit; a heat exchanger configured to transfer the heat generated by the combined heat and power unit to the heat medium; a first heat medium passage configured such that the heat is transferred to the heat medium by the heat exchanger and the heat medium flows into the heat accumulator; a heat medium supplier configured to cause the heat medium to flow through the first heat medium passage; a heat supplying passage through which the heat medium stored in the heat accumulator is supplied to a heat load; an electric power consuming heater configured to heat the heat medium flowing through the heat supplying passage toward the heat load by consuming surplus electric power of the combined heat and power unit and commercial electric power; and a second heat medium passage (10, 13, 15, 11) configured such that the heat medium is heated by the electric power consuming heater and flows into the heat accumulator, characterized in that the heat supplying passage through which the heat medium supplied from the heat accumulator reaches the electric power consuming heater constitutes a part of the second heat medium passage (10, 13, 15, 11) through which the heat medium having passed through the electric power consuming heater reaches the heat accumulator, the cogeneration system further comprising: a first heat medium supplying passage through which the heat medium is supplied from a heat medium source to the heat accumulator; a second heat medium supplying passage configured to branch off from the first heat medium supplying passage to be connected to the heat supplying passage; a mixing valve disposed on a portion where the heat supplying passage and the second heat medium supplying passage are connected to each other and configured to mix the heat medium flowing through the heat supplying passage with the heat medium from the second heat medium supplying passage; a bypass passage configured to connect the second heat medium passage (10, 13, 15, 11) and a portion of the heat supplying passage which portion is located on the heat accumulator side of the mixing valve; and a bypass valve disposed on the bypass passage, wherein the second heat medium passage (10, 13, 15, 11) is constituted by a passage extending from the heat accumulator through the first heat medium supplying passage, the second heat medium supplying passage, the bypass passage, and the heat supplying passage to the heat accumulator. a combined heat and power unit configured to generate electricity and heat; a heat accumulator configured to store a heat medium having recovered the heat generated by the combined heat and power unit; a heat exchanger configured to transfer the heat generated by the combined heat and power unit to the heat medium; a first heat medium passage configured such that the heat is transferred to the heat medium by the heat exchanger and the heat medium flows into the heat accumulator; a heat medium supplier configured to cause the heat medium to flow through the first heat medium passage; a heat supplying passage through which the heat medium stored in the heat accumulator is supplied to a heat load; an electric power consuming heater configured to heat the heat medium flowing through the heat supplying passage toward the heat load by consuming surplus electric power of the combined heat and power unit and commercial electric power; and a second heat medium passage (10, 13, 15, 11) configured such that the heat medium is heated by the electric power consuming heater and flows into the heat accumulator, a first heat medium supplying passage through which the heat medium is supplied from a heat medium source to the heat accumulator; a second heat medium supplying passage configured to branch off from the first heat medium supplying passage to be connected to the heat supplying passage; a mixing valve disposed on a portion where the heat supplying passage and the second heat medium supplying passage are connected to each other and configured to mix the heat medium flowing through the heat supplying passage with the heat medium from the second heat medium supplying passage; a bypass passage configured to connect the second heat medium passage (10, 13, 15, 11) and a portion of the heat supplying passage which portion is located on the heat accumulator side of the mixing valve; and a bypass valve disposed on the bypass passage, wherein the second heat medium passage (10, 13, 15, 11) is constituted by a passage extending from the heat accumulator through the first heat medium supplying passage, the second heat medium supplying passage, the bypass passage, and the heat supplying passage to the heat accumulator.