This paper describes an internal finishing process for thick non-ferromagnetic tube (10~20 mm in thickness) by the application of a magnetic field-assisted machining process using a magnetic machining jig (permanent magnet tool). In this study, a new automatic inner surface finishing system was developed, and to achieve smooth surface roughness and high form accuracy, a multiple-stage machining which contains of rough machining and precision finishing was carried out. Especially, in order to improve the form accuracy the rough processing time was made longer compared with the research in the past. The experiments were performed for a thick SUS304 stainless steel tube 10 mm in thickness. The results showed that surface-roughness and form accuracy were able to be improved greatly, the initial surface roughness of 4.9μm Ra can be improved to 0.01 μm Ra and the roundness of inside tube can be improved from 206 μm to13μm.