To improve response and versatility by A/D converting a data signal for the portion of one packet, counting the bit length of the data signal, successively adding bit lenght in the respective data signals within a unit time and calculating traffic quantity.

A signal receiving means 14 receives the Manchester-encoded data signal which s transmitted by a transmission path 1 for every packet. An A/D converting means 15 A/D-converts the data signal for the part of one packet, which is received by the signal receiving means 14. A bit length count means 16 successively counts the bit length of the data signal for the part of one packet, which is A/D-converted by the A/D converting means 15, at every unit time. A total bit lenght count means 19 successively counts the bit length of the data signal for the part of one packet, which is counted by the bit length counting means 16, every unit time so as to calculate traffic quantity. A display means 6 displays a traffic quantity display picture and also displays an alarm picture including a message at the time of alarm issuing.