The present invention enables accurate simulation of the state of communication over a communication network in which a communication channel of different communication characteristic is set for each protocol and session, such as a VoIP network built in a multi-vendor environment, and evaluation of communication quality of communications between IP phones, etc. over a supposed communication network. A relay device for relaying packet communications between a plurality of communication devices is used, and the relay device sets a virtual processing unit as an initial process so that it corresponds to the packet transmission direction and/or attribute, based on the contents of a received packet specified by a control protocol, and relays communications of packets after the initial process, based on the packet loss rate and delay time distribution indicated by communication state information associated with the processing unit.