Disclosed is a method for discharging lubricating oil in a system for reliquefying boil-off gas by compressing the boil-off gas by means of a compressor, subjecting the compressed boil-off gas to heat exchange with the pre-compression state boil-off gas in a heat exchanger, and thus cooling same, and decompressing, by means of a decompressing device, the fluid cooled by the heat exchange. According to the method for discharging lubricating oil, the compressor comprises at least one oil-lubricated cylinder, and it is determined that a "time to discharge condensed or coagulated lubricating oil" has come when at least one of the following conditions is satisfied: a condition where the difference between the temperature, at the front end of the heat exchanger, of the boil-off gas used as a refrigerant in the heat exchanger, and the temperature of the boil-off gas cooled by the heat exchanger after being compressed by the compressor ("temperature difference of low-temperature flow" hereafter) is maintained at a level equivalent to or higher than a first configuration value for a predetermined time or longer; a condition where the difference between the temperature of the boil-off gas used as a refrigerant in the heat exchanger, and the temperature of the boil-off gas transferred to the heat exchanger after being compressed by the compressor ("temperature difference of high-temperature flow" hereafter) is maintained at a level equivalent to or higher than the first configuration value for a predetermined time or longer; and a condition where the difference between the pressure, at the front end of the heat exchanger, of the boil-off gas transferred to the heat exchanger after being compressed by the compressor, and the pressure, at the rear end of the heat exchanger, of the boil-off gas cooled by the heat exchanger ("pressure difference of high-temperature flow path" hereafter) is maintained at a level equivalent to or higher than a second configuration value for a predetermined time or longer.