The optical heterodyne method is remarkable as a noncontact method that measures physical values with high precision. To achieve high precision, there are many restraints and conditions. In this chapter, first, the principle of the method is described, and the defects of the method are also shown. Second, the restraints and conditions are explained. These require the introduction of a cancelable optical circuit as a necessary condition. Third, several desirable conditions for optical parts and mechanical parts used in the circuit are given in detail. Finally, some applications are listed.