  Pions in external magnetic field are investigated in the frame of a
Pauli-Villars regularized Nambu--Jona-Lasinio model. The meson propagators in
terms of quark bubbles in Ritus and Schwinger schemes are analytically derived,
and pion masses are numerically calculated in the Ritus scheme. For neutral and
charged pions at finite temperature, there exist respectively one and three
mass jumps at the corresponding Mott transition points, due to the discrete
energy levels of the two constituent quarks in magnetic field.
