To indicate an installation method with an emphasis on balance from a viewpoint that a vertical shaft windmill is symmetric in a horizontal direction.

A center shaft of a double shaft is a fixed shaft for weight support, and bearings adapted to the fixed shaft are attached to both upper and lower ends of a rotary shaft of its outer side and the fixed shaft is inserted to form the double shaft. In an order of the installation method, when installing the center shaft in the construction of a base connecting the center shaft, a coupling or the like for the connection of a thrust bearing for weight support and the base are positively attached, and the rotary shaft is connected to the base after attaching arm-attaching parts, disk centers or the like and vertically erected. Alternatively, a method of inserting the center shaft from above by using a lift or the like after vertically erecting the center shaft. Anyhow, the construction method may be different depending on a device scale of the vertical shaft windmill such as a large scale and a small scale. In windmill assembling, assembling is symmetrically carried out with favorable balance from an upper portion by rotating it around the center shaft as a base point and in consideration of a scaffold.