This paper is a study on the bad effect of monopoly and analyzes seriousness of welfare loss caused by monopolization with firms data. The target for this analysis is statistical data of 291 companies among list stock companies of business groups that are issued by Korean Investors Service, Inc. This paper has a purpose to study what is the policy implications that results from determination through statistical data-based empirical analysis The result of this study is as the following, First, welfare loss caused by monopolization was 8.1% of shipping amount of manufacturing industry. So it is necessary to reinforce administrational consideration of regulating monopoly and promote competition to solve welfare loss problem caused by monopoly. Second, we can find that welfare cost is high when market scale is big. Namely, it is confirmed from this result that reinforced regulation for monopoly is necessary. 20 years or more has passed since government performed “Monopoly Regulation and Fair Trade Act” but there has been no remarkable achievement. So it is necessary to find alternative plan to raise efficiency for companies through modifying industrial structure to competitive structure, guaranteeing fair competition among firms in market and curtailing welfare cost.