In a system for controlling a headlight of a vehicle to be controlled, a dazzling level determiner determines a parameter value of a dazzling level of a driver of an other vehicle ahead of the controlled vehicle based on information associated with a relative positional relationship between the controlled vehicle and the other vehicle. The dazzling level represents a possibility that the driver of the other vehicle feels dazzling to light irradiated from the headlight of the controlled vehicle. A light-quantity adjuster adjusts a quantity of light to be irradiated from the headlight of the controlled vehicle based on the determined parameter value of the dazzling level.