We have investigated the magnetic field dependence of transfer characteristics of a device fabricated
in a configuration of a field-effect transistor with a conduction channel formed by a semiconducting
multiwalled carbon nanotube. Our results unambiguously indicate that an axial magnetic field
suppresses the band gap of the nanotube. Quantitative analysis of the data indicates linear
dependence of the band gap on magnetic field as well as a linear splitting between the K and K'
subbands of the band structure of the nanotube.