Speed change adjustment mechanism and headlamp device

A shift adjustment mechanism (1) is provided with: an adjustment unit (30) including a first slider (32); an adjustment target module (35) including a second slider (15); and a switching unit (40) including a first rotating shaft (41) and a link member (42) supported so as to be rotatable about the first rotating shaft (41) with respect to the fixed member (20). The link member (42) has a first engaged section (42a) that engages with the first engagement section (32a) of the first slider (32), and a second engaged section (42b) that engages with the second engagement section (15a) of the second slider (15). The switching means (40) moves at least one of the first rotating shaft (41), the first engagement section (32a), and the second engagement section (15a) so as to change at least one of a first distance (L1) between the first rotating shaft (41) and the first engagement section (32a), and a second distance (L2) between the first rotating shaft (41) and the second engagement section (15a). 变速调整机构(1)具有：调整单元(