To provide a traffic quality measuring instrument which can measure four service quality items of 'packet loss', 'response time', 'connection continuance time', and 'session continuance time' continuously at the same time by evaluating the quality of a protocol without exerting influence on the measured network or traffic.

Lines 105 and 107 between ATM exchanges 101 and 108 are led into the measuring instrument 111 through optical branching devices 104 and 106 without giving any changes to the traffic itself to measure the quality of 'packet loss', 'response time', 'connection continuance time', and 'session continuance time' continuously at the same time. The measuring instrument 111 comprises an I/O control part 117 to which a traffic gathering part 112, a filtering process part 113, a quality measurement part 114, a CPU 115, a memory 116, a monitor (display unit) 118, and a keyboard 119 are connected.