One aspect of the present invention provides a method for manufacturing a strain wave gear device. The method includes steps of measuring a between pin diameter of an internal gear, measuring an over pin diameter of an external gear, where the external gear is to be placed radially inside the internal gear, configured to mesh with the internal gear, and flexible, measuring a dimension of a bearing, where the bearing is to be placed radially inside the external gear, and flexible, and machining an elliptical cam, where the elliptical cam is to be placed radially inside the bearing, and configured to flex the external gear in a non-circular manner. The elliptical cam is machined based on the measured dimensions of the internal gear, external gear and bearing such that the internal and external gears mesh with each other at a constant position.