A heating device comprising: a heat source unit including a heating heat exchanger; and a circulation circuit in which an air-heating heat exchanger and a heat storage tank are connected together and through which a heating medium heated by the heating heat exchanger circulates, wherein the heating device further includes a controller configured to switch operation modes of the heating device from a first mode to a second mode, or vice versa, the first mode of operation including an operation of allowing, while the heat source unit is running, the heating medium heated by the heating heat exchanger to flow through the heat storage tank and then return to the heating heat exchanger, the second mode of operation including an operation of allowing, while the heat source unit is not running, the heating medium in the heat storage tank to flow through the air-heating heat exchanger, be bypassed around the heating heat exchanger, and then return to the heat storage tank, wherein the controller is configured to instruct the heating device to perform alternately and repeatedly a first operation and a second operation during the first mode of operation, the first operation being allowing, while the heat source unit is running, the heating medium heated by the heating heat exchanger to be bypassed around the heat storage tank, flow through the air-heating heat exchanger, and then return to the heating heat exchanger, the second operation being allowing, while the heat source unit is running, the heating medium heated by the heating heat exchanger to flow through both the heat storage tank and the air-heating heat exchanger and then return to the heating heat exchanger. a heat source unit including a heating heat exchanger; and a circulation circuit in which an air-heating heat exchanger and a heat storage tank are connected together and through which a heating medium heated by the heating heat exchanger circulates, wherein the heating device further includes a controller configured to switch operation modes of the heating device from a first mode to a second mode, or vice versa, the first mode of operation including an operation of allowing, while the heat source unit is running, the heating medium heated by the heating heat exchanger to flow through the heat storage tank and then return to the heating heat exchanger, the second mode of operation including an operation of allowing, while the heat source unit is not running, the heating medium in the heat storage tank to flow through the air-heating heat exchanger, be bypassed around the heating heat exchanger, and then return to the heat storage tank, wherein the controller is configured to instruct the heating device to perform alternately and repeatedly a first operation and a second operation during the first mode of operation, the first operation being allowing, while the heat source unit is running, the heating medium heated by the heating heat exchanger to be bypassed around the heat storage tank, flow through the air-heating heat exchanger, and then return to the heating heat exchanger, the second operation being allowing, while the heat source unit is running, the heating medium heated by the heating heat exchanger to flow through both the heat storage tank and the air-heating heat exchanger and then return to the heating heat exchanger.