Operation method for combined plant

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 (1) for generating steam and supplying the steam into a steam turbine (9), a condenser (10) for condensing steam passing through the steam turbine (9), a steam turbine bypass line (15) for connecting the upstream of the steam turbine (9) and the condenser (10) and bypassing the steam turbine (9); and a gas turbine steam cooling portion (6) for cooling a hot portion of the gas turbine plant by steam supplied from the steam drum (1), and which is provided parallel to a pipe connecting the upstream of the steam turbine bypass line (15) and the steam drum (1), wherein when the combined plant is in a normal operation mode, an amount of steam passing through the gas turbine steam cooling portion (6) is adjusted by a valve (16) which is provided in the steam turbine bypass line (15).