ROTATION ADJUSTMENT MECHANISM AND HEADLIGHT DEVICE

A rotation adjustment mechanism includes a rotation target module supported to be freely rotatable with respect to a fixation member around a first rotary shaft and an adjustment unit including a slider. Rotation of the rotation target module around the first rotary shaft is regulated by a contact point where a connection groove of the rotation target module and a projection part of the slider contact each other, and a shape of a surface of the connection groove contacting the projection part is a shape formed by a locus that successively connects the contact points of the connection groove and the projection part each of which is obtained each time the rotation target module is moved around the first rotary shaft by a unit angle and simultaneously the projection part fixed to the slider is translated by a constant amount in a direction parallel to the slide axis.