PURPOSE: To effectively utilize communication network resources and to improve communication quality by observing packet flow to be sent from a terminal to a packet exchange network by a packet flow observing means and calculating the statistic amount of a probability variable expressing the characteristic of the packet flow or the estimate value of a distribution function by a statistic arithmetic means.

CONSTITUTION: As the probability variable expressing the characteristic of the packet flow, time T1 from the arrival of a (i)th packet P1 to the arrival of a (i+1)th packet P1+1 is used. A statistic arithmetic means 2 observes the packet flow from a packet flow observing means 1 and calculates a sample value To of an average arrival interval T as the estimated value. A function arithmetic means 4 calculates a function S from an average value Tavo of the T to be applied from a distribution function setting means 3, distribution σ2 and the sample value To of the average arrival time. This function S is compared with a comparing area α from a comparing area setting means 5 by a deciding means 6. When the function value S is smaller than this α, it is decided the packet flow against a determined condition.