  By performing N-body simulations of chemodynamical evolution of galaxies with
dusty starbursts, we investigate photometric evolution of gas-rich major
mergers in order to explore the nature of ultraluminous infrared galaxies
(ULIRGs) with the total infrared luminosity ($L_{\rm IR}$ for $8\sim 1000$
$\mu$m) of $\sim$ $10^{12}$ $L_{\odot}$. Main results are the following three.
(1) Global colors and absolute magnitudes the during dusty starburst of a major
merger do not change with time significantly, because interstellar dust heavily
obscures young starburst populations that could cause rapid evolution of
photometric properties of the merger. (2) Dust extinction of stellar
populations in a galaxy merger with large infrared luminosity ($L_{\rm IR}$ $>$
$10^{11}$ $L_{\odot}$) is selective in the sense that younger stellar
populations are preferentially obscured by dust than old ones. This is because
younger populations are located in the central region where a larger amount of
dusty interstellar gas can be transferred from the outer gas-rich regions of
the merger. (3) Both $L_{\rm IR}$ and the ratio of $L_{\rm IR}$ to $B$ band
luminosity $(L_{\rm B}$) increases as the star formation rate increase during
the starburst of the present merger model, resulting in the positive
correlation between $L_{\rm IR}$ and $L_{\rm IR}/L_{\rm B}$.
