A steam turbine has a diffuser that is configured to guide steam to an outside of a casing. The diffuser has an outer guide that gradually expands to an outer side in a radial direction and an inner guide that is disposed at intervals to an inner side in the radial direction with respect to the outer guide. The inner guide has an inner curved diameter-expanded portion that gradually expands to the outer side in the radial direction while being curved from the first side to the second side in the axial direction. The outer guide has a first diameter-expanded portion that gradually expands to the outer side in the radial direction with a first radius of curvature, and a second diameter-expanded portion that gradually expands to the outer side in the radial direction with a second radius of curvature larger than the first radius of curvature.