A steam turbine, comprising: a high-pressure turbine, an intermediate-pressure turbine, and a low-pressure turbine; a boiler unit including an auxiliary boiler and a main boiler for supplying steam to the high-pressure turbine and the intermediate-pressure turbine; first supply pipes diverging from the auxiliary boiler and respectively extending to the high-pressure turbine and the intermediate-pressure turbine; a second supply pipe coupled with the high-pressure turbine, the intermediate-pressure turbine, and a condenser so that steam supplied through the first supply pipes is transferred to the condenser via the high-pressure turbine and the intermediate-pressure turbine; a plurality of valve units respectively disposed on each of the first supply pipes and configured to be operated when the auxiliary boiler is operated; a drain valve unit disposed on the second supply pipe and configured to discharge condensate water that has passed through the high-pressure turbine and the intermediate-pressure turbine; and a controller configured to control an opening amount of each of the valve units and the drain valve unit and to control an operation state of the boiler unit based on a first setting temperature, wherein the controller is configured to control, before rotation of the turbine starts, an operation time and temperature of the auxiliary boiler so that the temperatures of the high-pressure turbine and the intermediate-pressure turbine are increased to the first setting temperature, and the controller is configured to control, after rotation of the turbine starts and when the temperatures of the high-pressure turbine and the intermediate-pressure turbine are maintained at the first setting temperature, operation of the main boiler such that the temperature of the intermediate-pressure turbine reaches a second setting temperature while operation of the auxiliary boiler is interrupted, and control, when the temperature of the intermediate-pressure turbine is maintained at the second setting temperature, the operation of the main boiler such that steam is supplied only to the high-pressure turbine. a high-pressure turbine, an intermediate-pressure turbine, and a low-pressure turbine; a boiler unit including an auxiliary boiler and a main boiler for supplying steam to the high-pressure turbine and the intermediate-pressure turbine; first supply pipes diverging from the auxiliary boiler and respectively extending to the high-pressure turbine and the intermediate-pressure turbine; a second supply pipe coupled with the high-pressure turbine, the intermediate-pressure turbine, and a condenser so that steam supplied through the first supply pipes is transferred to the condenser via the high-pressure turbine and the intermediate-pressure turbine; a plurality of valve units respectively disposed on each of the first supply pipes and configured to be operated when the auxiliary boiler is operated; a drain valve unit disposed on the second supply pipe and configured to discharge condensate water that has passed through the high-pressure turbine and the intermediate-pressure turbine; and a controller configured to control an opening amount of each of the valve units and the drain valve unit and to control an operation state of the boiler unit based on a first setting temperature, wherein the controller is configured to control, before rotation of the turbine starts, an operation time and temperature of the auxiliary boiler so that the temperatures of the high-pressure turbine and the intermediate-pressure turbine are increased to the first setting temperature, and the controller is configured to control, after rotation of the turbine starts and when the temperatures of the high-pressure turbine and the intermediate-pressure turbine are maintained at the first setting temperature, operation of the main boiler such that the temperature of the intermediate-pressure turbine reaches a second setting temperature while operation of the auxiliary boiler is interrupted, and control, when the temperature of the intermediate-pressure turbine is maintained at the second setting temperature, the operation of the main boiler such that steam is supplied only to the high-pressure turbine.