An assembly method of assembling a substrate transfer device including: a transfer system unit forming step of fixing a robot and a substrate container retainer to a divided body which composes a part of the substrate transfer device and is formed separably on a main structural body as a residual part of the substrate transfer device, thereby forming a transfer system unit; an operation examination step of examining whether the robot fixed to the transfer system unit can operate as a part of the substrate transfer device or not; and a mounting step of mounting the transfer system unit on the main structural body of the substrate transfer device after the operation examination step.