To provide a congestion determining device that allows the operation load to be reduced on the occasion of congestion determining execution at an information center.

In a congestion determining device, from a preceding vehicle traveling in front of a subject vehicle, information on a preceding vehicle group consisting of the preceding vehicle and other vehicles traveling further ahead of the preceding vehicle is acquired. Based on the information on the preceding vehicle group, information on a subject vehicle group consisting of the subject vehicle and vehicles belonging to the preceding vehicle group is calculated, and also it is determined whether or not there exists, in the backward direction of the subject vehicle, a transmission object vehicle fitted for an object for the information on the subject vehicle group to be transmitted. When it is determined that the transmission object vehicle exists, the information on the subject vehicle group is transmitted to the transmission object vehicle. When it is determined that no transmission object vehicle exists, it is determined whether or not congestion is generated based on the information on the subject vehicle group. If it is determined that the congestion is generated, the information on the subject vehicle group is transmitted to an information center.