A covert system is in objective to hide critical information. If a company has a covert system, it may have had illegible information. Who has it? Why? Most companies want to make good progress of debt finance, inducement of investment and to get a good company image. If the company doesn't get a good condition on its finance and management, it commits some frauds. The company manipulates financial statements in order to get good assessment results about the state of its finance and management from investors and manages actual data in a covert system. When the company's fraud comes out and the investigation launches, the management may want to secrete his covert system. On the step of data acquisition, the covert system detection influences on the investigation results. If the investigator couldn't detect the covert system and the fraud company having a covert system may not say about it, the data on the covert system may not come to the front. It is difficult for the investigator to find out the covert system on the physical place or the network. So it is required a field guide to detection of a covert system with meeting the situation of the investigation field. This paper introduces the field investigation guide for detecting the covert system having accounting information and the detection skill according to the types of the covert system.