PURPOSE: To trace the traffic of calls of a specific data terminal address even at any station by forming a designated data of trace objective call as a terminal address and a trace position as a packet processing section, in a packet exchange.

CONSTITUTION: A start circuit 40 of a packet exchange 4 gives start/stop command and deleting command of storage data respectively to a discriminating circuit 42 and a storage circuit 44. The circuit 42 sets a data terminal device address by the start command, a header section of a packet of a line corresponding section 41 and a line number added by the section 41 are compared, and when they are coincident, the circuit 42 stores the line number and transfers the content of storage to a line 44 and transmits the packet itself to a packet processing section 43. When the result of comparison of line numbers is dissident, the circuit 42 transmits a packet from the corresponding section to a processing section 43 as it is. The processing section 43 uses the line number and traces the call of the terminal device.