The present invention relates to a device (roadside relay device 2) configured to generate a traffic index used for traffic signal control. This device includes: a storage unit 24 configured to store therein area information on a coordinate system, the area information forming a predetermined area on a road; a communication unit 21 configured to receive probe information S5 including a vehicle position and temporal information of a traveling vehicle 5; and a control unit 23 configured to generate the traffic index on the basis of the area information and the probe information S5.