For each of the traveling directions (traveling direction 1 to traveling direction 4) at an intersection, a traffic-light cycle length estimation device acquires a time at which a vehicle in the stopped state starts moving, calculates the time difference between neighboring start times, which have been acquired, as a start interval, and generates a histogram based on the number of samplings of start intervals. The device combines the generated histograms into a histogram for all direction to generate one histogram that represents the relation between the start intervals and the number of samplings and, based on this histogram, estimates the cycle length of the traffic light. If a particular value, one of the start intervals, corresponds to the maximum number of samplings, that particular value is estimated as the cycle length.