Driving force transmission mechanism

A driving force transmission mechanism (22) is equipped with a first gear (44) attached to a drive source (20), a second gear (46) enmeshed with the first gear (44), and a damper shaft (50) enmeshed mutually with the second gear (46) and with a rack member (32) of an air mixing damper (18). A first locator part (56) formed on the second gear (46) is inserted in a first locator groove (54a) in the first gear (44), while additionally, a second locator part (66) is inserted with respect to a second locator groove (88) in a damper gear (82). Further, a first narrow tooth (58) formed on the first gear (44) enmeshes with a meshing groove (70a) of the first locator part (56), whereas a second narrow tooth (90) formed on the damper gear (82) enmeshes with another meshing groove (70b) of the second locator part (66).