To restrain the reduction of the ratio of signal to noise(S/N) due to the reflecting wave from the inner face of a vacuum vessel by covering the inner face of the vacuum vessel in which a microwave detecting element is arranged with a reflection preventing film.

The opening part of a vacuum vessel 1 made of stainless steel is airtightly blockaded with a glass window 2. The inner face of the vacuum vessel 1 is mirror polished, and covered with a black body forming processed film 3 (reflection preventing film). The black body forming processed film 3 is a film or the like containing chromium formed by quenching, for example, a nickel oxide film, a chromium boride film, or stainless steel at about 400°C and depositing chromium on the surface. A refrigerator 5 is arranged inside the vacuum vessel 1, and a superconducting antenna 4 is fitted to the cold head 5a. By treating the inner face of the vacuum vessel in this way, reflection of microwave incident into the vacuum vessel 1 can be reduced, and S/N of received microwave by the superconducting antenna 4 is improved.