The invention relates to an expansion machine (20), comprising an output shaft (24) and a shaft sealing ring (25) that interacts with the output shaft. The expansion machine (20) has an inflow region (21) and an outflow region (22). During operation, a working medium flows through the expansion machine (20), wherein compressed working medium flows into the inflow region (21) and expanded working medium flows out of the outflow region (22). The shaft sealing ring (25) separates a valve space (11) filled with working medium from a surrounding space (40). A valve (10) is arranged in the expansion machine (20). The pressure in the valve space (11) can be controlled by means of the valve (10).