  We show that the 750 GeV diphoton excess can be explained by introducing
vector-like quarks and hidden fermions charged under a hidden U(1) gauge
symmetry, which has a relatively large coupling constant as well as a
significant kinetic mixing with U(1)$_Y$. With the large kinetic mixing, the
standard model gauge couplings unify around $10^{17}$ GeV, suggesting the grand
unified theory without too rapid proton decay. Our scenario predicts events
with a photon and missing transverse momentum, and its cross section is related
to that for the diphoton excess through the kinetic mixing. We also discuss
other possible collider signatures and cosmology, including various ways to
evade constraints on exotic stable charged particles. In some cases where the
750 GeV diphoton excess is due to diaxion decays, our scenario also predicts
triphoton and tetraphoton signals.
