Computer security breaches are already a major problem in using computers. The most basic defense against it is to monitor and audit the computer logs. Computer logs, however, have a huge amount of textual data. It is, therefore, almost impossible to inspect them manually using current systems. We propose a log visualization system called "Tudumi". Tudumi consists of several functions which assist system administrators to perform such taksmanually. These functions are information visualization, log summarization and reflecting known rules into the visualization mehthod. Tudumi makes it easier to detect anomalous user activities, such as intrusion, from a huge amount of computer logs.