  We calculate axisymmetric spheroidal modes of neutron stars magnetized with
an axisymmetric poloidal magnetic field. We use polytropes of the indices
$n\sim1$ as background equilibrium models of neutron stars where we ignore the
deformation due to the magnetic fields. For a poloidal magnetic field,
axisymmetric normal modes of non-rotating stars are decoupled into spheroidal
modes and toroidal modes, and we can treat spheroidal modes separately from
toroidal modes. For the surface field strength $B_S$ ranging from $10^{14}$G to
$10^{16}$G, we calculate axisymmetric spheroidal magnetic modes whose
oscillation frequency is proportional to $B_S$. The typical oscillation
frequency of the magnetic modes is $\sim 0.01\times\sqrt{GM/R^3}$ for $B_S\sim
10^{15}$G, where $M$ and $R$ are respectively the mass and radius of the star
and $G$ is the gravitational constant. For $M=1.4M_\odot$ and $R=10^6$cm, this
frequency is $\sim 20$Hz, which may explain low frequency QPOs found for SGR
1806-204 and SGR 1900+14. We also find modes of frequency $> \sqrt{GM/R^3}$
corresponding to the radial fundamental and first harmonic modes. No unstable
magnetic modes are found for axisymmetric spheroidal oscillations of magnetized
stars.
