In cartel practice, business actors form an agreement on the establishment of a cartel, in which the agreement is made verbally, not written. In revealing the cartel practice, KPPU finds difficulties in finding written evidence about the cartel establishment, therefore KPPU uses indirect evidence to reveal cartel practices. Indirect evidence itself is the result of analysis in the form of indications that there has been agreement on the establishment of cartel, but did not explain clearly how the deal took place. Indirect evidence could be form as evidence of communication and economic evidence, where economic evidence is divided into two forms, evidence of behavior and structural evidence. In practice, indirect evidence is commonly used by KPPU to reveal cartel practices, but in business competition law in Indonesia, indirect evidence is not regulated as an evidence that can reveal cartel practices. This conditions inspired the writer to do research about hthe legality of indirect evidence as an evidence in business competition law in Indonesia and how its application in cartel cases. In analyzing the legality of indirect evidence and its application, the writer used case number 10 / KPPU-I / 2015 about imported beef as reference. In the case, the Commission Assembly sentenced 32 reported parties that proven guilty of violating the provisions of Article 11 juncto Article 19 letter c of Law Number 5 Year 1999 based on indirect evidence. This paper uses normative legal research methods to assess the legality of indirect evidence and its application using Statue Approach and Cases Approach. From the research, the writer has identified that indirect evidence can be categorized as an indication, where the indication is a knowledge of Commission Assembly that are believed to be true. In practice, the indication can not be the only evidence, but must be presented at least with two other direct evidence.