excessive to the contrary according to the present invention the generated refrigeration quantity is calculated to select the elastic force of the tension ring 28 so as to reduce the leakage of the helium gas and generate refrigeration for example when the sliding resistance was set to be 4 of the indicated refrigeration quantity an improved sealability was obtained on the other hand a quantity of leakage of the helium gas is dependent on a surface roughness of the inner circumferential surface of the cylinder 10 fig6 shows a relationship between the surface roughness of the inner surface of the cylinder 10 and the attainable temperature of the third thermal stage 20 when the surface roughness of the inner surface of the cylinder 10 was set to 0 5 u03bcm rms the attainable temperature was 3 68k fig5 c shows a preferred embodiment using the third seal 7b of a labyrinth type a clearance between an outer circumferential surface of the labyrinth seal 7b and an inner circumferential surface of the cylinder 10 is made very small to thereby increase the resistance upon passing of the helium gas 16 therethrough and reduce the quantity of the helium gas 16 passing therethrough furthermore as the sliding resistance of the labyrinth seal 7b is small the heat.