The official name of the Antimonopoly Act is the Act on Prohibition of Private Monopolization and Maintenance of Fair Trade. The Act sets the rules that companies must follow in the society of a free economy. Under the Act, conducts that prevent fair, and free competition are regulated.

As you can see, the structure of the Antimonopoly Act can be summarized as in the above.
From a consumer standpoint, we would lose benefits such as the opportunity to choose better quality, more inexpensive products if there was no competition between companies in the markets. The Antimonopoly Act prohibits companies from conducting such acts.
In addition to the Antimonopoly Act, the Fair Trade Commission enforces the Subcontract Act (official name: the Act against Delay in Payment of Subcontract Proceeds, Etc. to Subcontractors) to supplement the Antimonopoly Act. To ensure fair and free competition in the markets, the Fair Trade Commission enforces these two Acts.