Provided is a steam turbine plant activation control device that can flexibly handle an initial state amount of a steam turbine plant and activate a steam turbine at a high speed. The activation control device 21 for the steam turbine plant includes a heat source device 1 configured to heat a low-temperature fluid using a heat source medium and generate a high-temperature fluid, a steam generator 2 for generating steam by thermal exchange with the high-temperature fluid, a steam turbine 3 to be driven by the steam, and adjusters 11, 12, 13, 14, 15 configured to adjust operation amounts of the plant.