A steam turbine assembling method includes: disposing an upper half partition plate having an upper half partition plate division surface on an inner peripheral side of an upper half casing having an upper half casing division surface to form an upper half assembly; and, after disposing a lower half partition plate having a lower half partition plate division surface capable of abutting against the upper half partition plate division surface on an inner peripheral side of a lower half casing having a lower half casing division surface capable of abutting against the upper half casing division surface, fixing a lower half position defining portion to the lower half partition plate in a state where a lower half abutment surface abuts against the lower half casing division surface and the lower half partition plate division surface to form a lower half assembly.