  We develop a spinor equation of the electromagnetic field, which is
equivalent to the Maxwell equation and has a similar form as the Dirac
equation. The spinor is the very conjugate momentum of the vector potential in
the Lagrangian mechanics. In this framework the electromagnetic field described
by the spinor exhibits the SU(2) internal symmetry. The quantization and the
problem of longinal and scalar photons are disscussed.
