PURPOSE: To provide a sphere machining device, in which a shape evaluating method based on a cross sectional shape data for a grinding wheel outer circumference part is use, by arranging a cross sectional shape measuring means in the width direction of a rotary grinding wheel and a means forming an outer circumference part, of the rotary grinding wheel in the vicinity of a main spindle mechanism rotationally driving an object to be machined.

CONSTITUTION: An outer circumference part 3a of a machining grinding wheel 3 is formed into a circular arc cross section by means of a truing device 7. Then, beams are irradiated to the outer circumference part, 3a of the grinding wheel 3 after truing by means of a cross sectional shape sensor 8, so that the cross sectional shape, deflection, and surface roughness of the grinding wheel are measured. when a target value is not satisfied, formation by means of the turning device 7 is carried out again. Subsequently, a radius of curvature is found on the basis of a circular arc cross section data, and on the basis of this radius of curvature, an offset is given to a machining NC offset program. On the basis of the obtained NC program, an objet 1 to be machined which is chucked in a main spindle 2 and is rotationally driven is ground. In this process, the grinding wheels are replaced until the predetermined finish roughness can be provided and these processes are repeated.