  Ultraluminous infrared galaxies (ULIRGs) have several types according to
dominance of starburst or AGN component. We made stellar population analysis
for a sample of 160 ULIRGs to study the evolution of ULIRGs. We found that the
dominance of intermediate-age and old stellar populations increases along the
sequence of HII-like ULIRGs, Seyfert-HII composite ULIRGs, and Seyfert 2
ULIRGs. Consequently the typical mean stellar age and the stellar mass increase
along the sequence. Comparing the gas mass estimated from the CO measurements
with the stellar mass estimated from the optical spectra, we found that gas
fraction is anti-correlated with the stellar mass. HII-like ULIRGs with small
stellar masses do not possess enough gas and the total mass, and therefore have
no evolution connections with massive Seyfert 2 ULIRGs. Only massive ULIRGs may
follow the evolution sequence toward AGNs, and massive HII-like ULIRGs are
probably in an earlier stage of the sequence.
