1. A behavior control system which controls the behavior of an agent according to a behavior plan, the behavior control system comprising an electronic control unit, and the electronic control unit comprising: a first arithmetic processing element configured to determine a predicted position trajectory of an object expressed by a probability density distribution of time-series predicted positions of the object in the future on the basis of time-series measured positions or estimated positions of the object; a second arithmetic processing element configured to generate a position trajectory candidate of a counter object expressed by a probability density distribution which indicates likelihoods corresponding to time-series positions of a counter object which includes a position coinciding, in a spatial and time-series manner, with an interaction point candidate included in the predicted position trajectory of the object in a range reachable by the counter object by a motion of the agent; and a third arithmetic processing element configured to generate a behavior plan such that a position trajectory of the counter object coincides with a desired position trajectory of the counter object, which is a mean position trajectory or a central position trajectory of one position trajectory candidate selected from among a plurality of position trajectory candidates of the counter object on the basis of the degree of overlapping of a time-series probability density distribution with the predicted position trajectory of the object.