When the digital forensic investigator looks into the suspicious organization including the company, it is important to obtain the data which has the overview of information of the organization. Since system structure of computer and network about the organization is complicated, the investigator who does not know its structure has a difficulty to detect the database system which has the data. In particular, if the organization intentionally builds the covert database system in order to hide data, then the investigator would have a more difficulty. In this paper, we describe structures of the covert database system for hiding fraud information in the organization. Next, we show techniques and propose our tool for detecting the covert database system. And then we suggest scenarios about investigation procedure using our tool.