A gas turbine cogeneration system comprising: a gas turbine; a heat recovery steam generator, a bypass stack provided upstream of the heat recovery steam generator in a gas turbine exhaust gas path; a bypass damper that opens/closes the bypass stack; and a control device that controls the gas turbine, the heat recovery steam generator, and the bypass damper, whereby the heat recovery steam generator includes: a duct burner; a main shut-off valve that is provided in a fuel line of the duct burner; a fuel shut-off valve that is provided upstream of the main shut-off valve in the fuel line of the duct burner; an air supply piping that joins the fuel line at a position upstream of the main shut-off valve and downstream of the fuel shut-off valve; an air supply shut-off valve that is provided in the air supply piping; a ventilation piping that is branched off from the fuel line at a position upstream of the main shut-off valve and downstream of the fuel shut-off valve; and a ventilation shut-off valve that is provided in the ventilation piping, characterized in that the control device is configured, during an operation using the bypass stack, to close an inlet of the heat recovery steam generator to open the bypass stack, close the main shut-off valve and the fuel shut-off valve, and always open the air supply shut-off valve and the ventilation shut-off valve, and at a time of changing to an operation using the heat recovery steam generator, to open the inlet of the heat recovery steam generator to close the bypass stack without shut down of the gas turbine, open the main shut-off valve and the fuel shut-off valve V, and close the air supply shut-off valve and the ventilation shut-off valve. a gas turbine; a heat recovery steam generator, a bypass stack provided upstream of the heat recovery steam generator in a gas turbine exhaust gas path; a bypass damper that opens/closes the bypass stack; and a control device that controls the gas turbine, the heat recovery steam generator, and the bypass damper, whereby the heat recovery steam generator includes: a duct burner; a main shut-off valve that is provided in a fuel line of the duct burner; a fuel shut-off valve that is provided upstream of the main shut-off valve in the fuel line of the duct burner; an air supply piping that joins the fuel line at a position upstream of the main shut-off valve and downstream of the fuel shut-off valve; an air supply shut-off valve that is provided in the air supply piping; a ventilation piping that is branched off from the fuel line at a position upstream of the main shut-off valve and downstream of the fuel shut-off valve; and a ventilation shut-off valve that is provided in the ventilation piping, characterized in that a duct burner; a main shut-off valve that is provided in a fuel line of the duct burner; a fuel shut-off valve that is provided upstream of the main shut-off valve in the fuel line of the duct burner; an air supply piping that joins the fuel line at a position upstream of the main shut-off valve and downstream of the fuel shut-off valve; an air supply shut-off valve that is provided in the air supply piping; a ventilation piping that is branched off from the fuel line at a position upstream of the main shut-off valve and downstream of the fuel shut-off valve; and a ventilation shut-off valve that is provided in the ventilation piping, characterized in that the control device is configured, during an operation using the bypass stack, to close an inlet of the heat recovery steam generator to open the bypass stack, close the main shut-off valve and the fuel shut-off valve, and always open the air supply shut-off valve and the ventilation shut-off valve, and at a time of changing to an operation using the heat recovery steam generator, to open the inlet of the heat recovery steam generator to close the bypass stack without shut down of the gas turbine, open the main shut-off valve and the fuel shut-off valve V, and close the air supply shut-off valve and the ventilation shut-off valve. during an operation using the bypass stack, to close an inlet of the heat recovery steam generator to open the bypass stack, close the main shut-off valve and the fuel shut-off valve, and always open the air supply shut-off valve and the ventilation shut-off valve, and at a time of changing to an operation using the heat recovery steam generator, to open the inlet of the heat recovery steam generator to close the bypass stack without shut down of the gas turbine, open the main shut-off valve and the fuel shut-off valve V, and close the air supply shut-off valve and the ventilation shut-off valve.