A phase operator formalism is presented within the framework of thermo field dynamics (TFD). It is shown that a unitary phase operator (or phase factor) can be defined as a canonical conjugate of the generator of phase shift in TFD since the generator has a lower bounded spectrum. The tilde conjugation plays an essential role in defining the unitary phase operator in TFD. The unitary phase operator is expressed in terms of the relative-number states. The properties of the unitary phase operator are investigated in detail. The relations to the Pegg-Barnett phase operator and to the polar decomposition of the operator which appears in the heterodyne detection are found. Furthermore, by making use of the phase operator method, a quantum phase measurement is considered and the relation to the optimal probability operator measure in the conventional theory is investigated.
