A group III nitride based semiconductor device which has a trench or mesa structure and of which leakage of current and reduction of breakdown voltage are prevented. A GaN layer 2 was grown on a C-plane sapphire substrate 1, and a T-shaped USG film 3 was formed on the GaN layer 2 so that side surfaces of the USG film 3 were arranged parallel to A-plane and M-plane of the GaN layer 2. Thereafter, by using the USG film 3 as a mask, the GaN layer 2 was dry-etched. As is clear from FIGS. 2A and 2B, the M-plane is less roughened as compared with the A-plane. Subsequently, wet-etched was performed by use of an aqueous TMAH solution. As is clear from FIGS. 2C and 2D, roughness of the A-plane and the M-plane are removed, and, particularly, the M-plane assumes a mirror surface. Thus, through provision of M-plane side surfaces of a trench or an etching-formed mesa, leakage of current and reduction of breakdown voltage of a group III nitride based semiconductor device can be prevented.