An object is to provide a light-irradiation technique that are capable of making the driver of a vehicle around a vehicle platoon aware of changes in the vehicle configuration of the vehicle platoon. During no changes in the vehicle configuration of a vehicle platoon, a controller performs control in such a manner that an irradiation device of the vehicle casts light in a steady-state mode common in the vehicle platoon. In a process where the vehicle merges with another of the vehicle alone or another of the vehicle platoon, the controller performs control in such a manner that the irradiation device of the merging vehicle casts light in a transition mode. In a process where the vehicle leaves the vehicle platoon, the controller performs control in such a manner that the irradiation device of the leaving vehicle casts light in the transition mode.