A heat source device comprising: a heat medium circulation passage which has a heating terminal connected on a way thereof and through which a heat medium circulates; a heat medium heating unit (11, 26) which is provided on a way of the heat medium circulation passage and which heats a heat medium that circulates in the heat medium circulation passage; a hot water supply bypass passage which bypasses the heat medium heating unit (11, 26) and which is in communication with the heat medium circulation passage; a hot water supply passage which has an upstream end thereof connected to waterworks and a downstream end thereof connected to a hot water supply tap; a hot water supply heat exchanger which is connected on a way of the hot water supply bypass passage and the hot water supply passage and which heats water flowing through the hot water supply passage by heat exchange between the heat medium flowing through the hot water supply bypass passage and water flowing through the hot water supply passage; a circulation mode switching unit which switches between a first circulation mode and a second circulation mode, the first circulation mode being a mode in which the circulation of the heat medium passing through the hot water supply bypass passage and the heat medium heating unit (11, 26) is disabled, whereas the circulation of the heat medium passing through the heat medium heating unit (11, 26) and the heating terminal is enabled, and the second circulation mode being a mode in which the circulation of the heat medium passing through the heating terminal and the heat medium heating unit (11, 26) is disabled, whereas the circulation of the heat medium passing through the hot water supply bypass passage and the heat medium heating unit (11, 26) is enabled; a control unit which sets the first circulation mode by the circulation mode switching unit and performs a heating operation, in which the heat medium heating unit (11, 26) is actuated, in response to a predetermined request for heating, and which sets the second circulation mode by the circulation mode switching unit and performs a hot water supply operation, in which the heat medium heating unit (11, 26) is actuated, in response to a predetermined request for hot water supply, characterized in that a hot water supply stop alarm unit which performs a hot water supply stop alarm to urge a user of hot water to stop using the hot water in a case where the request for heating is continued for a first predetermined time or more while the hot water supply operation is being performed. a heat medium circulation passage which has a heating terminal connected on a way thereof and through which a heat medium circulates; a heat medium heating unit (11, 26) which is provided on a way of the heat medium circulation passage and which heats a heat medium that circulates in the heat medium circulation passage; a hot water supply bypass passage which bypasses the heat medium heating unit (11, 26) and which is in communication with the heat medium circulation passage; a hot water supply passage which has an upstream end thereof connected to waterworks and a downstream end thereof connected to a hot water supply tap; a hot water supply heat exchanger which is connected on a way of the hot water supply bypass passage and the hot water supply passage and which heats water flowing through the hot water supply passage by heat exchange between the heat medium flowing through the hot water supply bypass passage and water flowing through the hot water supply passage; a circulation mode switching unit which switches between a first circulation mode and a second circulation mode, the first circulation mode being a mode in which the circulation of the heat medium passing through the hot water supply bypass passage and the heat medium heating unit (11, 26) is disabled, whereas the circulation of the heat medium passing through the heat medium heating unit (11, 26) and the heating terminal is enabled, and the second circulation mode being a mode in which the circulation of the heat medium passing through the heating terminal and the heat medium heating unit (11, 26) is disabled, whereas the circulation of the heat medium passing through the hot water supply bypass passage and the heat medium heating unit (11, 26) is enabled; a control unit which sets the first circulation mode by the circulation mode switching unit and performs a heating operation, in which the heat medium heating unit (11, 26) is actuated, in response to a predetermined request for heating, and which sets the second circulation mode by the circulation mode switching unit and performs a hot water supply operation, in which the heat medium heating unit (11, 26) is actuated, in response to a predetermined request for hot water supply, characterized in that a hot water supply stop alarm unit which performs a hot water supply stop alarm to urge a user of hot water to stop using the hot water in a case where the request for heating is continued for a first predetermined time or more while the hot water supply operation is being performed.