Sub-headlight unit, headlight unit, headlight system for vehicles that lean into turns, a vehicle that leans into turns and a method for controlling a sub-headlight unit for vehicles that lean into turns

A sub-headlight unit (13L,13R) that can desirably illuminate inside of a curve regardless of whether the low beam or the high beam is on, while suppressing a size increase. The sub-headlight unit (13L,13R) for a vehicle (10) that leans into turns include a sub headlight light source (13La-c,13Ra-c) that provides illumination forward and outward of the width direction of the vehicle (10) on one side of the width direction of the vehicle (10), wherein the sub headlight light source (13La-c,13Ra-c) turns on when the lean angle of the vehicle (10) leaning to the one side of the width direction of the vehicle (10) reaches a low-beam reference value (KL) while a low beam light source (11L) of the vehicle (10) is on, and wherein the sub headlight light source (13La-c,13Ra-c) turns on when the lean angle of the vehicle (10) leaning to the one side of the width direction of the vehicle (10) reaches a high-beam reference value (KH) smaller than the low-beam reference value (KL) while a high beam light source (11H) of the vehicle (10) is on.