A computer system for realizing increased speed of identifying extent of a failure in a messaging system, provided with: a first computer including a message receiving part, a first log output part, and a first memory part configured to store receiving log data; a second computer including a data store management part configured to manage a data store, a first search part configured to search a message that meets a given condition from among messages stored in the data store, a second log output part, and a second memory part configured to store data store log data; a third computer including a message sending part, a third log output part, and a third memory part configured to store sending log data; and a fourth computer including a monitoring part, a log collecting part, and a second search part configured to search for lost message.