A headlight device includes: a light source that emits light; a light guide element that receives the light emitted from the light source through an incident surface and guides the received light to emit the guided light from an emitting surface; and a radiation optical system that radiates the light emitted from the emitting surface ahead of a vehicle. The light guide element includes: a first light guide portion that extends from the incident surface to the emitting surface and guides the received light; and a second light guide portion that is in contact with the first light guide portion, extends from the incident surface to the emitting surface, and guides the received light. The first and second light guide portions have different refractive indexes. The light guide element is configured so that part of light entering the first light guide portion can enter the second light guide portion.