PURPOSE: To quickly and exactly execute the judgement of a traffic snarl by judging whether a traffic flow is snarled or not by comparing the data of the present time with the mean value of the data of the past.

CONSTITUTION: Various kinds of sensor data are calculated by transmitting a time series presence/absence pulse train obtained by an ultrasonic wave sensor 11 in respect of a vehicle 10 passing through a spot to a sensor data calculating part 12. Various kinds of the sensor data such as passing traffic volume or an occupancy rate or mean velocity, etc., obtained from the sensor data calculating part 12 are stored by a sensor data storage part 130, and in a past data mean value calculating part 131, the mean value of mean velocity data in the comparatively near past among the various kinds of the sensor data of the past stored by the sensor data storage part 130 is calculated, and difference between the mean value and the mean velocity data of the present time is calculated by a difference calculating part 132, and a threshold inputted beforehand by a threshold inputtiong part 133 and the difference are compared by a comparing and judging part 134, and it is judged whether the traffic flow is snarled or not.