In a disc axis adjusting mechanism in a gas turbine, the gas turbine has an exhaust gas diffuser provided between a casing wall and a bearing case which are connected to a downstream side of a turbine, a plurality of struts provided at intervals in a circumferential direction, and strut covers coupling an outer diffuser and an inner diffuser of the exhaust gas diffuser so as to cover the struts, and the mechanism comprises: a plurality of air introduction holes formed in the casing wall so as to allow the interior and exterior of the wall to communicate with each other; a sensor unit configured to detect a parameter corresponding to the thermal elongation of each of the struts; and a flow rate adjustor configured to adjust the flow rate of air flowing through each of the air introduction holes based on a detection value detected by the sensor unit.