This study is an investigation of Axial-Inection End-Burning Hybrid Rockets aimed at revealing fuel regression characteristics under relatively high pressure conditions. Firing tests were conducted using gaseous oxygen as oxidizer at chamber pressures and oxidizer port velocities ranging from 0.22 MPa to 1.05 MPa and 31 m/s to 103 m/s, respectively. The results of fifteen static firing tests show that fuel regression rate increases as the chamber pressure increases, and regression rates ranged from approximately 1.1 mm/s at 0.25 MPa to 5.4 mm/s at 0.71 MPa. Furthermore, it is observed that the fuel regression rate is not influenced by oxidizer port velocity. The athours encountered a problem refered to as backfiring in this paper, and developed a calculation model to investigate this problemanalytically. The calculation model explains why the back-firing problem tends to occur in relatively high-pressure conditions, and leads to the conclusion that increasing nozzle throat diameter is an effective means of preventing back-firing from occuring.