PURPOSE: To omit the spacers and improve the heat conduction efficiency by recessing the fitting portion of the first heat stage on the side face of a bottomed cylindrical cryo-panel to form a flat fitting face, and fitting the first heat stage to this fitting face in the heat contact state.

CONSTITUTION: The fitting portion of the first heat stage 14 is recessed on the side face of the bottomed cylindrical first cryo-panel 7 to form a flat fitting face 11, and the first heat stage 14 is directly fitted to the fitting face 11 in the heat contact state. Spacers pinching the cryo-panel 7 from both sides to fit the heat stage 14 are not required, the weight can be reduced, the total length can be shortened, and the structure can be miniaturized.