  We revisit the adiabatic conversion between the QCD axion and axion-like
particle (ALP) at level crossing, which can occur in the early universe as a
result of the existence of a hypothetical mass mixing. This is similar to the
Mikheyev-Smirnov-Wolfenstein effect in neutrino oscillations. After refining
the conditions for the adiabatic conversion to occur, we focus on a scenario
where the ALP produced by the adiabatic conversion of the QCD axion explains
the observed dark matter abundance. Interestingly, we find that the ALP decay
constant can be much smaller than the ordinary case in which the ALP is
produced by the realignment mechanism. As a consequence, the ALP-photon
coupling is enhanced by a few orders of magnitude, which is advantageous for
the future ALP and axion-search experiments using the ALP-photon coupling.
