Assembly method of wind power generation system

Provided is an assembly method of a wind power generation system that can enhance the safety of an assembly work, and which can reduce a working time. In order to solve the above problem, the assembly method of a wind power generation system of the present invention includes assembly of the wind power generation system which includes a rotor 3 with a hub 1 and blades 2, a nacelle 7 for accommodating therein at least a generator 6 connected to the rotor 3 via a main shaft 4 connected to the hub 1, and a tower 9 supporting the nacelle 7 on a top portion thereof, and having an opposite side thereof to the top portion fixed to a foundation 8, the tower 9 including separated tower parts. When assembling the wind power generation system, the nacelle 7 and the tower 9 are laterally assembled together by using a carriage 10, and the rotor 3 is fixed to the laterally-facing nacelle 7.