Abstract: A flushing method for a lubricating oil system includes a step of performing precedent inside oil flushing for bearing box, by connecting an oil supply pipe and a flushing apparatus mounted to a bearing box to each other by a bypass pipe while bypassing an internal pipe that is provided in the bearing box and introduces an oil supplied from the oil supply pipe to a part between a rotor of a turbine and a bearing, supplying the oil from the oil supply pipe to the flushing apparatus through the bypass pipe, and injecting the oil from the flushing apparatus into the bearing box, to thereby perform oil flushing for the inside of the bearing box; and a final oil flushing step of supplying the oil from the oil supply pipe to an internal pipe, to thereby perform oil flushing for the inside of the lubricating oil system, after the step of performing precedent inside oil flushing for bearing box.