A horizontally split casing of a turbo machine, such as a centrifugal compressor, including one half casing formed with scroll grooves on an inner peripheral surface thereof, and the other half casing formed with cutouts in portions thereof corresponding to outlet scrolls. A second casing formed on an inner peripheral surface thereof with scroll grooves is attached to the other half casing in a manner to enclose the cutouts. The horizontally split casing of this construction is low in cost and yet enables a compact size and a light weight to be obtained in a horizontally split casing of a turbo machine.