  We have shown a new production mechanism of radiations produced by dark
matter axion $a(t)$ under strong magnetic field $B$. The axion generates
oscillating electric current in the surface of conductor. The current is
extremely enhanced such as $J_c=(m_a\delta_e)^{-2}J_a$ compared with vacuum
current $J_a\equiv g_{a\gamma\gamma}\partial_t a(t)B$; $\delta_e$ is skin depth
and axion mass $m_a$. We have shown that sufficiently strong radiation is
emitted by cylindrical super ( normal ) conductor to be easily detected. Then,
we naively expect that such strong radiation also arises in resonant cavity
experiment. But we show that the expectation is wrong. The radiation generated
in the cavity is just the one given in the standard estimation, even if we
consider the enhanced current in the conductor of the cavity. This is because
the radiation in the cavity is standing wave, while the radiation emitted from
the cylindrical conductor is outgoing wave. For simplicity we consider
radiation arising in the space between two parallel flat conductors.
