Since Ion sputter machining can perform removing processing in atom or molecule units in vacuum state, it has the merit that high precision processing is possible. In this study, therefore, the effect of incidence ion beam is certified to processing amount and surface roughness when longtimed processing is applied. As a result, processing amount is made almost constant with time and the best processing condition is achieved when the incidencial angle of ion is 55.deg.. In addition, processing time for the good surface roughness is different respectively to the quality of material and longtimed processing has some defect for achieving good surface roughness.