Magneto-optical spectra of a single-wall carbon nanotube are studied theoretically. For light polarization perpendicular to the tube axis, spectral peaks are strongly diminished by a depolarization field. In a high perpendicular magnetic field, however, clear spectral peaks remain at higher energy side from interband transition energy. For parallel polarization, the dynamical circumference current providing the depolarization field, is expected in a perpendicular magnetic field. However, the depolarization field does not appear unless an Aharonov-Bohm flux is applied.