We analyze domain name system (DNS) errors (i.e., Serv- Fail, Refused and NX Domain errors) in DNS traffic cap- tured at an external connection link of an academic network in Japan and attempt to understand the causes of such er- rors. Because DNS errors that are responses to erroneous queries have a large impact on DNS traffic, we should reduce as many of them as possible. First, we show that ServFail and Refused errors are generated by queries from a small number of local resolvers and authoritative nameservers that do not relate to ordinary users. Second, we demonstrate that NX Domain errors have several query patterns due to mostly anti-virus/anti-spam systems as well as meaningless queries (i.e., mis-configuration). By analyzing erroneous queries leading to NX Domain errors with the proposed heuristic rules to identify the main causes of such errors, we suc- cessfully classify them into nine groups that cover approxi- mately 90% of NX Domain errors with a low false positive rate. Furthermore, we find malicious domain names similar to Japanese SNS sites from the results. We discuss the main causes of these DNS errors and how to reduce them from the results of our analysis.