To provide a method of assembling door modules, which is capable of improving an operating efficiency by simplifying an operation of assembling the door modules.

A holder is prepared in advance which is switchable between a predetermined first position and a second position which is approximately inverted from the first position; a lift of a window regulator and a guide rail are positioned and held in predetermined positions at the holder in the first position (a first process); a module plate is positioned, fixed, and held at the holder so that a rail mounting part of the module plate is overlapped at the guide rail (a second process); the holder is moved to the second position so that a wheel for driving a wire connected to the lift is assembled to the module plate (a third process); the holder of the second position is moved to the first position so that a electric motor for driving the wheel is mounted at the module plate (a fourth process).