Brightness profile data having a number of dimensions is extracted based on image data in a radial direction of an optical fiber, the brightness profile data representing features for each rotation angle of the optical fiber. Machine learning uses training data to create a prediction model that based on the brightness profile data determines a rotation angle of each pair of optical fibers is. The pair of optical fibers are rotated to the determined rotation angle and then fusion spliced. The training data indicates a correspondence relationship between the rotation angle of the optical fiber and brightness profile in the radial direction for each rotation angle of the optical fiber. The prediction model can determine a rotation angle of an arbitrary optical fiber based on brightness profile data of the arbitrary optical fiber.