To provide redundancy to a link mechanism provided in a steam turbine. A backup device for a link mechanism includes a stand-by plate suspended from a regulating valve lever as a first link and located immediately below a contact stopper provided on a main rod as a second link. The backup device preferably further includes a clamp that is provided on the regulating valve lever and suspends the stand-by plate. If the links are disjoined due to damage to a joint that joins the links of the link mechanism and thus the main rod is disengaged from the regulating valve lever by its own weight, the contact stopper provided on the main rod is locked by the stand-by plate located immediately below the contact stopper.