  We investigate the influence of an external magnetic field on chiral symmetry
breaking in the Nambu-Jona-Lasinio (NJL) model at finite temperature and
chemical potential. According to the Fock-Schwinger proper-time method, we
calculate the effective potential in the leading order of the $1/N_c$
expansion. The phase boundary dividing the symmetric phase and the broken phase
is illustrated numerically. A complex behavior of the phase boundary is found
for large chemical potential.
