We study the adiabatic conversion between the QCD axion and axion-like particle (ALP) at level crossing
which occurs when their masses become close to each other in the early universe. This is similar to the
Mikheyev-Smirnov-Wolfenstein effect in neutrino oscillations. I explain a scenario where the ALP produced
by the adiabatic conversion of the QCD axion explains the observed dark matter abundance. Interestingly,
the ALP-photon coupling is enhanced by a few orders of magnitude, which is advantageous for the on-going
and future axion search experiments using the axion-photon coupling.