The present invention provides an operation method of a plant which has a low-pressure feed water heater, a deaerator and a high-pressure feed water heater sequentially arranged in a feed water pipe reaching a steam generator from a condenser, and leads high-temperature feed water to the steam generator, wherein an oxidant is injected onto a surface of a structural material from an oxidant injection line in order to form a film that suppresses an elution of an element constituting the structural material such as the feed water pipe, the low-pressure feed water heater, the deaerator and the high-pressure feed water heater, which come in contact with the high-temperature feed water, and a corrosion suppression substance is further introduced from a corrosion suppression substance introduction line in order to deposit the corrosion suppression substance on a surface of the structural material in which corrosion accelerated by a flow of the feed water occurs.