To facilitate replacement of a beam of a guard pipe using inner sleeves.

There is provided a method of replacing the beam of the guard pipe, which is composed of the following steps. In the detaching step, a specific beam 51 is detached from the guard pipe. In the insertion step, a first inner member is detached from a connector and a beam adjacent to the specific beam, and inserted into one end of a replacement beam to be newly introduced. In the mounting step, while a second inner member is inserted into an end of the replacement beam, that is opposite to the end into which the first inner member is inserted, the end into which the first inner member is inserted is held in a position opposed to the corresponding connector, and the first inner member is moved to the connector in the opposed position. Thus both the ends of the first inner member are protruded from the connector, and insertable into the replacement beam and the beam adjacent to the same. In such a state, both the ends of the first inner member are fastened to the beams into which they have been inserted.