  We report detection of a line-like feature in the $\gamma$-ray spectrum of
the blazar B0516$-$621, for which the data obtained with the Large Area
Telescope onboard {\it Fermi Gamma-Ray Space Telescope (Fermi)} areanalyzed.
The feature is at $\sim$7\,GeV and cannot be explained with radiation processes
generally considered for jet emission of blazars. Instead, it could be a signal
due to the oscillations between photons and axion-like particles (ALPs) in the
source's jet. We investigate this possibility by fitting the spectrum with the
photon-ALP oscillation model, and find thatthe parameter space of ALP mass
$m_a\leq 10^{-8}$\,eV and the coupling constant (between photons and ALPs)
$g_{a\gamma}$=1.17--1.49$\times10^{-10}$\,GeV$^{-1}$ can provide a fit to the
line-like feature, while the magnetic field at the emission site of
$\gamma$-rays is fixed at 0.7\,G. The ranges for $m_a$ and $g_{a\gamma}$ are in
tension with those previously obtained from several experiments or methods, but
on the other hand in line with some of the others. This spectral-feature case
and its possible indication for ALP existence could be checked from similar
studies of other blazar systems and also suggest a direction of effort for
building future high-energy facilities that would have high sensitivities and
spectral resolutions for searching for similar features.
