To avoid actuation of a safety valve, or rupture of a rupture disk at the time of emergency stop of a steam turbine.

A bypass valve regulation means 15B to change and regulate opening of a bypass valve Va based on steam pressure on the upstream side of a steam turbine 3 is provided, so the bypass valve Va starts opening when steam pressure exceeds a set value, a steam quantity detection means 17 and an operator 16 are provided, the operator 16 changes and regulates opening of the bypass valve Va corresponding to output of the bypass valve regulation means 15B during normal operation of the steam turbine 3, it quickly and forcedly set opening of the bypass valve Va to that in which similar or roughly similar quantity of steam to inflow steam quantity to the steam turbine 3 immediately before emergency stop flows at the time of emergency stop of the steam turbine 3, and after emergency stop, it gradually returns opening of the bypass valve Va to that corresponding to output of the bypass valve regulation means 15B.