To obtain an optical axis adjusting mechanism which is of simple structure and is easily assembled, and in which two different optical axes are crossed at a desired position or both of them are made parallel.

The optical axis adjusting mechanism is equipped with: a lens barrel unit 1 equipped with a photographic optical system; and a finder unit 2 attached to the unit 1 and also equipped with a finder optical system being an optical system different from the photographic optical system, and adjusts the relative positional relation of each of the optical axes X and Y of both optical systems to the desired positional relation. The mechanism is equipped with a fulcrum part 31 being a turning fulcrum for relatively turning the units 1 and 2, optical axis relative positioning means 5 and 7 making the relative turning of the units 1 and 2 impossible at a plurality of specified relative turning positions with the fulcrum part 31 as the turning fulcrum and positioning the relative positional relation of the respective optical axes X and Y of both optical systems at any position out of a plurality of relative turning positions, and a fixing means 6 fixing the unit 1 and the unit 2 in a state where the units 1 and 2 are positioned by the positioning means 5 and 7.