To provide a single shaft type gas turbine system capable of optimizing the number of revolutions of a gas turbine or a rotary machine to be driven by the gas turbine by corresponding to load fluctuation of the rotary machine driven by the gas turbine.

This single shaft type gas turbine system S is provided with the single shaft type gas turbine 1, the rotary machine driven by the single shaft type gas turbine 1, a continuously variable transmission 3 provided between the gas turbine 1 and the rotary machine, and a controller 4. The number of revolutions of the gas turbine 1 is controlled to the optimum number of revolutions by an optimum fuel consumption amount map M and the controller 4, and a change gear ratio of the continuously variable transmission 3 is controlled in such a way that the number of revolutions of the rotary machine becomes desired number of revolutions.