We first review current status of QCD axion searches, which generically
rely on interaction between the axion and SM photon.
Next, we show that the axion-photon coupling is modified in the presence
of massless hidden photon which has kinetic mixing with the SM photon. If
the kinetic mixing and hidden gauge coupling are \mathcal{O}(1), the
axion-photon coupling becomes several tens of times larger.
Finally, we consider QCD axion in SU(5) GUT with massless hidden photon.
Gauge coupling unification is determined by the size of kinetic mixing
only, and requires relatively large kinetic mixing, which is naturally
generated when hidden gauge coupling is large. We find enhancement factor
of axion-photon coupling is about 20 in our model.