An inner casing (10) of a rotating thermal machine, in particular a steam turbine, has essentially the form of a hollow cylinder and is divided in a parting plane (20), which extends through an axis (11) of said casing, into a lower casing part (12) and an upper casing part (13), which casing parts (12, 13) are detachably connected to one another in the parting plane (20). The thermomechanical properties are improved with respect to casings provided with flanges in that the two casing parts (12, 13) have, in the region of the parting plane (20), two overlapping areas (14, 15) which lie opposite one another and in which the two casing parts (12, 13) overlap one another and are detachably connected to one another with overlapping sections (17, 18), in that the two casing parts (12, 13) have a predefined wall thickness (d), and in that the sum of the thicknesses of the overlapping sections (17, 18) is approximately the same as the wall thickness (d) of the two casing parts (12, 13) in the overlapping areas (14, 15).