To finish the working of the internal spherical surface form of a work at a high sphericity accuracy.

The axis L2 of a rotary shaft at the supporting device side of a work is crossed to the axis L1 of a spindle at a U-shaft working machine side, and the edge 9a of a tool 9 is fitted to the rotation center O of the work. The tool 9 is moved in the U-shaft direction so that the rotating locus of the edge 9a draws a sectional circle 43a, and the spindle is rotated, and as a result, parts S1 and S1 are cut in to projection ends 42 and 42a respectively. Then, by rotating the work 90° making the L2 as the shaft from that part, the upper half and the lower half of the sectional circle 43a are to draw the locus of 1/4 of a spherical form 43 relatively, to the projection ends 42 and 42a, respectively, and parts S2 and S2 interfering with them are cut off at point- simmetrically respectively. When the work is rotated 90° further, the sector form part of remaining 1/4 are cut off, and the parts 44 and 44 of the internal spherical surface form interferring with the spherical form 43 are cut off respectively, at the opposing surfaces of the projection ends 42 and 42a.