This paper presents a new method for separate measurement of rotational motion errors and a radius circumference profile of a disc plane in the main rotational axis of a manufacturing machine. This method, called the mixed method, uses three displacement sensors and one angle sensor. By measuring a certain radius circumference of a disc plane using these sensors and applying a calculation technique, it is possible to separate axial motion error, two angular motion errors, and the circumference profile. In the present paper, the principle of the mixed method is examined from the viewpoint of spatial frequency analysis. From this principle, it appears that this new method can measure all spatial frequencies, without omitting any frequency. In contrast, the 3-point method, which has also been as an error separation method, has characteristics whereby some frequencies are inevitably omitted. We also set up a measurement system and performed experiments. The measurement experiments confirmed the separate measurement of the profile and motion errors.