To provide an irradiation device that more properly strikes a balance between visibility for a driver and glare prevention properties, and to provide an irradiation method, a program and a medium.

The irradiation device 1 includes: a detector 2a for detecting positions of other vehicles running ahead of a vehicle; a classification determiner 2b for determining whether the other vehicle is an oncoming vehicle or a leading vehicle; a belongingness determiner 2c for determining whether the position of the oncoming vehicle belongs to any one of the predetermined one or more kinds of prescribed areas; a setting device 2d for setting a prescribed range that is other than a prescribed area determined that it belongs to and configured to correspond to the prescribed area; and a controller 2e for controlling an irradiation range of a forward area of the vehicle. The controller 2e changes the irradiation range by proportionally moving the range with the position of the leading vehicle when the other vehicle is determined to be the leading vehicle, and also fixes the irradiation range within the prescribed range when the other vehicle is determined to be the oncoming vehicle and the position of the oncoming vehicle is determined that it belongs to the prescribed area.