  Axion-like particles (ALPs) can oscillate to photons and vice versa in
electromagnetic fields. The photon-ALP oscillation provides an attractive
solution to the apparent transparency of the Universe to TeV photons. The
allowed parameter regions for the ALP mass $m_{\rm a}\leq 10^{-7}$ eV have been
tightly constrained by the Fermi-LAT and H.E.S.S observations of some
extragalactic sources. In this work we show for the first time that the H.E.S.S
observations of some TeV sources in the Galactic plane exclude the highest ALP
mass region (i.e., $m_{\rm a}\sim {\rm a ~few\times 10^{-7}}$ eV) that accounts
for the TeV transparency of the Universe. The upcoming CTA observations of the
Galactic TeV sources are shown to be able to improve the constraints
significantly.
