We have performed a magnetotransport study on an AlGaN/GaN heterostructure at low temperatures. The effective-mass values have been evaluated by analyzing the exact form of the temperature-dependent Shubnikov-de Haas oscillation function. The values obtained increase with the magnetic field. This mass enhancement is attributed to conduction-band nonparabolicity. The effective-mass variation with the magnetic field was extrapolated to zero field, together with further correction due to the triangular confinement of the carriers, yielding an effective mass of 0.185 ± 0.005 of the free-electron mass. Our result is in excellent agreement with the results obtained by first-principle calculations and the tight-binding method, and suggest the significance of magnetic-field-induced nonparabolicity in transport measurements.