clear boundary is present between the transparent portion 1a and 1c and the translucent or opaque portion 1b the transmittance of the ceiling portion 1a of the vessel 1 was by far more than 85 and more 90 or more in some measured points the transmittance of the side portion 1b was on the contrary much lower than 40 and in some points 10 or less fig1 is a schematic view illustrating the operational principle of an apparatus for manufacturing a reactor vessel 1 as shown in fig1 having area 1d which is halfway in transmittance between the transparent ceiling portion la and the translucent or opaque side portion 1b special suction holes 10d in which gas permeable carbon is filled are formed along the lower end of the side portion 10b adjacent to the bottom portion 10a to realize a gradual change in bubble densities bubble content in the half level region were 20 000 bubbles cm 3 or more when bubbles having diameters dispersed in the range of 10 u03bcm to 250 u03bcm were counted and the transmittance of heat was measured at values of 30 or less which satisfies a condition of the present invention fig9 is a schematic view illustrating the internal construction of a single.