PURPOSE: To attain a multi cast in a multistage network consisting of plural exchange switches that perform the packet exchanges by preparing a buffer for the multi cast and also applying a special control system in a multi cast mode.

CONSTITUTION: A multi cast buffer is added to an exchange switch of a conventional system to newly store a packet of the largest length. Then the multi cast buffer is used in such a case where the packets cannot be outputted to one or more ports via a switch when a multi cast is carried out in a multistage network using the Wormhole routing as shown in a diagram. As a result, the packets are sent to the output enable ports and at the same time the packt data are stored in the multi cast buffer. Then the data are taken out of the multi cast buffer and transferred when the output unable ports are turned into the output enable states.