An operation method for a combined plant can simplify the steam lines and also decrease the cost for manufacturing combined plants as well. The combined plant comprises a gas turbine plant and a steam turbine plant; the steam turbine plant comprising a steam drum for generating steam and supplying the steam into a steam turbine, a condenser for condensing steam passing through the steam turbine, a steam turbine bypass line for connecting the upstream of the steam turbine and the condenser and bypassing the steam turbine; and a gas turbine steam cooling portion for cooling a hot portion of the gas turbine plant by steam supplied from the steam drum, and which is provided parallel to a pipe connecting the upstream of the steam turbine bypass line and the steam drum, wherein when the combined plant is in a normal operation mode, an amount of steam passing through the gas turbine steam cooling portion is adjusted by a valve which is provided in the steam turbine bypass line.