An encryption error monitoring system for checking packets transmitted between a private network and an external network. The system includes includes a detector module (60) which is connected to receive the packet for determining whether or not the packet is successfully encrypted in accordance with a specific security protocol such as IPSec (Internet Protocol Security). The detector module reads from the packet a sender's address and a destination address, and provides an error signal when the packet is judged not to be successfully encrypted. A manager module (80) is connected to the detector module within the private network to create, upon receipt of the error signal, a report including the sender's address and the destination address with regard to the packet judged not to be successfully encrypted. The detector module is configured to have a judge means (66) which reads a header of the packet and checks whether or not the header includes a protocol code that matches with a particular code identifying the specific security protocol. When the header does not include the protocol code in match with the particular code, the judge means provides the error signal to notify the encryption error. Thus, the encryption error can be determined only by referring to the unencrypted header and therefore without necessitating the decryption of the packet.