An assembly or disassembly method for a casing of a steam turbine in which an inner casing is installed in an outer casing, and the inner casing and the outer casing are fixed at a fixing position, wherein when the inner casing is assembled or disassembled in a state where the fixing position and a gravity center of the inner casing are at different positions in an axial direction of the inner casing, a tilt adjusting jig is interposed between the outer casing and the inner casing so that the assembly or disassembly of the casing is performed while maintaining a tilt of the inner casing with the tilt adjusting jig.