  We reconsider the conversion of the photon into axion in the external
electromagnetic fields, namely in the static fields and in a periodic field of
the wave guide. The total cross-sections for the conversion are evaluated in
detail. The result shows that with strong strength of external electromagnetic
fields, the cross-sections are large enough to measure the axion production. In
the wave guide there exists the resonant conversion at the low energies, in
which the value of cross-sections is much enhanced
