With increasing population densities in cities, underground facilities and entertainment locations are proliferating at a rapid pace in efforts to maximize the use of land. The purpose of this study is to evaluate CO concentration level of subway stations having underground platforms in Busan Metropolitan City, from September to November 2000, over seven times. The places of inquiry include Yonsan-dong station, Somyon station, Busan station, Nampo-dong station and Dusil station. The samplings were conducted at three points of each station, i.e. gates, ticket gates, and platforms. The component of CO concentration is the main elements of indoor air quality evaluation. The total concentrations and average concentrations of CO were the higher at the Somyon station. The distribution of CO concentrations at each station, generally was higher at gate than ticket gates and platforms. The I/O ratios(I-platforms, 0-gates) were expressed highly than 1 at the Yonsan-dong station, Busan station, Nampo-dong station and were lower than 1 at the others.