  We propose a mechanism of producing a new type of primordial perturbations
that collapse to primordial black holes whose mass can be as large as necessary
for them to grow to the supermassive black holes observed at high redshifts,
without contradicting COBE/FIRAS upper limits on cosmic microwave background
(CMB) spectral distortions. In our model, the observable Universe consists of
two kinds of many small patches which experienced different expansion histories
during inflation. Primordial perturbations large enough to form primordial
black holes are realized on patches that experienced more Hubble expansion than
the others. By making these patches the minor component, the rarity of
supermassive black holes can be explained. On the other hand, most regions of
the Universe experienced the standard history and, hence, only have standard
almost-scale-invariant adiabatic perturbations confirmed by observations of CMB
or large-scale structures of the Universe. Thus, our mechanism can evade the
constraint from the nondetection of the CMB distortion set by the COBE/FIRAS
measurement. Our model predicts the existence of supermassive black holes even
at redshifts much higher than those observed. Hence, our model can be tested by
future observations peeking into the higher-redshift Universe.
