The present invention relates to a leak reducing structure in a steam turbine having high pressure, intermediate pressure, and low pressure turbine sections in a single casing, in which steam leaking from the high pressure side to the intermediate pressure side is recovered to be used effectively. The high pressure, intermediate pressure, and low pressure turbine sections are arranged along a rotor in an external casing. High-pressure steam from a high-pressure steam inlet port passes through a nozzle chamber formed integrally with a dummy ring, and flows into the high pressure turbine section to do work. On the other hand, some of high-pressure steam attempts to leak from a seal portion of the dummy ring to the intermediate pressure turbine section side. However, the leaking steam flows from point X of an external pipe to point Y on the high pressure side to be recovered. Point X is set so as to be a slightly higher pressure point than point Y, and the difference in pressure is regulated by a pressure regulating valve. Therefore, the leaking high-pressure steam is recovered and used effectively, thereby preventing the decrease in performance.