A headlight device includes an optical unit to emit light and a support mechanism to support the optical unit. The optical unit includes supported pins as a first supported part and supported groove parts as a second supported part. The support mechanism includes support groove parts as a first support part that is provided on a first straight line in a predetermined first direction and contacts the supported pins and support pins as a second support part that is provided on a second straight line in a second direction orthogonal to the first direction and contacts the supported groove parts. The support mechanism supports the optical unit to be freely rotatable around the first straight line passing through the support groove parts and freely rotatable around the second straight line passing through the support pins.