A cleaning method of a heat sterilization system comprising a heating section for heating a content and a cooling section to which a beverage is sent from the heating section, the heating section having a heating pipe with a fluid inlet (13a 1) and a fluid outlet (13a2), wherein the heating section and the cooling section are connected to each other by a hot-water line through which a medium from the cooling section is supplied to the heating section and connected by a cooling-water line through which the medium from the heating section is supplied to the cooling section, and wherein the hot-water line is provided with a heated-steam supply unit configured to supply heated steam into the hot-water line, the cleaning method comprising: a step of causing a fluid including at least a cleaning liquid to flow through the heating pipe of the heating section so as to clean an inside of the heating pipe of the heating section by means of a fluid circulation line; a step of heating the heating pipe of the heating section from outside by means of the medium; a step of measuring fluid temperatures of the fluid including at least the cleaning liquid at the fluid inlet (13a 1) and the fluid outlet (13a2) of the heating pipe; a step of measuring medium temperatures of the medium at a medium inlet and a medium outlet of the heating section; a step of measuring a flowrate of fluid circulating the fluid circulation line; a step of calculating an amount of heat in the heating section based on the fluid temperatures of the fluid including at least the cleaning liquid at the fluid inlet (13a 1) and the fluid outlet (13a2) of the heating pipe, and the flowrate; and a step of calculating an overall heat transfer coefficient of the heating pipe and monitoring the overall heat transfer coefficient, based on the fluid temperatures of the fluid including at least the cleaning liquid at the fluid inlet (13a 1) and the fluid outlet (13a2) of the heating pipe, the medium temperatures of the medium at the medium inlet and the medium outlet of the heating section, the amount of heat in the heating section, and a heat transfer area of the heating pipe, wherein when the overall heat transfer coefficient exceeds a desired value, the cleaning of the inside of the heating pipe is finished, or a cleaning condition of the heating pipe heat sterilization system is determined based on the overall heat transfer coefficient. a step of causing a fluid including at least a cleaning liquid to flow through the heating pipe of the heating section so as to clean an inside of the heating pipe of the heating section by means of a fluid circulation line; a step of heating the heating pipe of the heating section from outside by means of the medium; a step of measuring fluid temperatures of the fluid including at least the cleaning liquid at the fluid inlet (13a 1) and the fluid outlet (13a2) of the heating pipe; a step of measuring medium temperatures of the medium at a medium inlet and a medium outlet of the heating section; a step of measuring a flowrate of fluid circulating the fluid circulation line; a step of calculating an amount of heat in the heating section based on the fluid temperatures of the fluid including at least the cleaning liquid at the fluid inlet (13a 1) and the fluid outlet (13a2) of the heating pipe, and the flowrate; and a step of calculating an overall heat transfer coefficient of the heating pipe and monitoring the overall heat transfer coefficient, based on the fluid temperatures of the fluid including at least the cleaning liquid at the fluid inlet (13a 1) and the fluid outlet (13a2) of the heating pipe, the medium temperatures of the medium at the medium inlet and the medium outlet of the heating section, the amount of heat in the heating section, and a heat transfer area of the heating pipe, wherein when the overall heat transfer coefficient exceeds a desired value, the cleaning of the inside of the heating pipe is finished, or a cleaning condition of the heating pipe heat sterilization system is determined based on the overall heat transfer coefficient.