Burning experiments were conducted to better understand the fuel regression behavior in a swirling-injection end-burning hybrid rocket engine using paraffin wax/gaseous oxygen propellant. The oxidizer mass flow rate, grain diameter, and the distance between the oxidizer injector and the grain end-surface were the variable parameters taken as influencing the regression rate. The engine attained an overall axial regression rate as high as approximately 5 mm/s, whereas unstable combustion occurred with increasing burning time owing to low melting temperature of paraffin wax. The fuel grain with a diameter of 90 mm also resulted in unstable combustion caused by the initial shallow crack of the cast grain. The radial distribution of the local regression rate exhibited dependency on the radial position and had two peaks: close to the periphery and the middle of the chamber. From the analogy of the heat transfer at the end surface in a vortex flow chamber, the controlling parameter of the overall axial regression rate was derived. Since this parameter depends on the distance between the oxidizer injector outlet and the fuel grain end-surface, to hold the grain end-surface by using some kind of actuator is necessary to attain constant O/F combustion.