  When the inflaton is coupled to the gluon Chern-Simons term for successful
reheating, mixing between the inflaton and the QCD axion is generally expected
given the solution of the strong CP problem by the QCD axion. This is
particularly natural if the inflaton is a different, heavier axion. We propose
a scenario in which the QCD axion plays the role of the inflaton by mixing with
heavy axions. In particular, if the energy scale of inflation is lower than the
QCD scale, a hybrid inflation is realized where the QCD axion plays the role of
the inflaton in early stages. We perform detailed numerical calculations to
take account of the mixing effects. Interestingly, the initial misalignment
angle of the QCD axion, which is usually a free parameter, is determined by the
inflaton dynamics. It is found to be close to $\pi$ in simple models. This is
the realization of the pi-shift inflation proposed in previous literature, and
it shows that QCD axion dark matter and inflation can be closely related. The
heavy axion may be probed by future accelerator experiments.
