  The recent discovery of high-redshift (z > 6) supermassive black holes (SMBH)
favors the formation of massive seed BHs in protogalaxies. One possible
scenario is formation of massive stars ~ 1e3-1e4 Msun via runaway stellar
collisions in a dense cluster, leaving behind massive BHs without significant
mass loss. We study the pulsational instability of massive stars with the
zero-age main-sequence (ZAMS) mass Mzams/Msun = 300-3000 and metallicity Z/Zsun
= 0-0.1, and discuss whether or not pulsation-driven mass loss prevents massive
BH formation. In the MS phase, the pulsational instability excited by the
epsilon-mechanism grows in ~ 1e3 yrs. As the stellar mass and metallicity
increase, the mass-loss rate increases to < 1e-3 Msun/yr. In the red
super-giant (RSG) phase, the instability is excited by the kappa-mechanism
operating in the hydrogen ionization zone and grows more rapidly in ~ 10 yrs.
The RSG mass-loss rate is almost independent of metallicity and distributes in
the range of ~ 1e-3-1e-2 Msun/yr. Conducting the stellar structure calculations
including feedback due to pulsation-driven winds, we find that the stellar
models of Mzams/Msun = 300-3000 can leave behind remnant BHs more massive than
~ 200-1200 Msun. We conclude that massive merger products can seed monster
SMBHs observed at z > 6.
