Disclosed herein is a dual processing system capable of ensuring real-time processing in a protocol conformance test. A protocol testing device performs a test on a layer under protocol test provided in a device under test. A communication device processes the protocol of a layer below the layer under protocol test between the device under test and the protocol testing device. The communication device processes a protocol test message, requiring real-time processing, instead of the protocol testing device, and transmits processing results for the protocol test message to the protocol testing device. The protocol testing device processes a protocol test message, not requiring real-time processing, and determines the conformance of the layer under protocol test provided in the device under test based on the processing results for the protocol test message.