PURPOSE: To shorten the starting time of a plant by recovering a medium in a plant system and retaining its heat while the plant is stopped.

CONSTITUTION: A heat insulating tank 6 sufficiently applied with a heat reserving work is provided in a solar thermal plant such as a distribution type. This heat insulating tank 6 has a capacity to recover a medium in the plant system. A drain valve 7 is opened when the plant is stopped operating. Then, a drain pump 8 is operated to recover the medium in the plant into the heat insulating tank 6. When the plant is to be started, a water filling pump 9 is operated to return the medium into the plant system.