  A simple slave-boson representation combined with the Hartree-Fock
approximation for the Hund's rule coupling is introduced for a doubly
degenerate narrow band, which bears a direct relation to that introduced
previously in the nondegenerate case. Namely, one keeps the fermion
representation of the spin operator to recover properly the energy of fermionic
quasiparticles in the presence of an applied magnetic field. A simple
two-parameter mean-field analysis of the metamagnetism is provided, with the
emphasis on the role of the Hund's rule coupling. We also analyse the
appearance of the spin-split effective masses in the applied field and for
nonhalf-filled-band situation. The Mott-Hubbard boundary is determined at
nonzero temperature (T>0); it shifts towards lower interactions with increasing
T and the field signalling the precursory localization effects, explicitly
exhibited in the behavior of the magnetic susceptibility calculated in the
Appendix. We also formulate a more general two-parameter rotationally invariant
approach for an arbitrary degeneracy d of equivalent orbitals and show that the
Mott-Hubbard transition at zero temperature and at any integer filling n>1 is
always discontinuous. A brief overview of experimental situation is also made.
