  A potential mechanism is proposed to account for the fact that supermassive
black holes (SMBHs) in disk galaxies appear to be smaller than those in
elliptical galaxies in the same luminosity range. We consider the formation of
SMBHs by radiation drag (Poynting-Robertson effect), which extracts angular
momentum from interstellar matter and thereby drives the mass accretion onto a
galactic center. In particular, we quantitatively scrutinize the efficiency of
radiation drag in a galaxy composed of bulge and disk, and elucidate the
relation between the final mass of SMBH and the bulge-to-disk ratio of the
galaxy. As a result, it is found that the BH-to-galaxy mass ratio, $M_{\rm
BH}/M_{\rm galaxy}$, decreases with a smaller bulge-to-disk ratio, due to the
effects of geometrical dilution and opacity, and is reduced maximally by two
orders of magnitude, resulting in $M_{\rm BH}/M_{\rm galaxy}\approx 10^{-5}$.
In contrast, if only the bulge components in galaxies are focused, the
BH-to-bulge mass ratio becomes $M_{\rm BH}/M_{\rm bulge}\approx 10^{-3}$, which
is similar to that found in elliptical galaxies. Thus, it turns out that the
mass of SMBH primarily correlates with a bulge, not with a disk, consistently
with observational data.
