A lens mounting mechanism comprising: a lens holder configured to hold an image pickup lens; and a case configured to have an insertion hole, in which the lens holder is inserted, formed in the case, wherein the lens holder is configured to include: an insertion part for being inserted into the insertion hole of the case in an insertion direction which is an optical axis direction of the image pickup lens, and a regulation part for regulating the insertion, wherein engaging parts (252, 352) are provided on a surface opposed to the case of the regulation part of the lens holder and a surface opposed to the regulation part of the case, and the engaging parts are formed so that the position of the lens holder relative to the case is adjustable in a direction perpendicular to the optical axis direction in a state where the engaging parts are engaged with each other; wherein the engaging part on the surface of the regulation part has a plurality of concentric ring-shaped projections formed therein surrounding the insertion part, and the engaging part on the surface of the case has a plurality of concentric ring-shaped projections formed therein surrounding the insertion hole, and then, the plurality of projections of the engaging part are inserted between the projections of the other engaging part so that the engaging parts are engaged with each other, and an interval of the projections of the engaging part on the surface opposed to the case is formed wider than that of the projections of the engaging part on the surface opposed to the regulation part, and an interval of the projections of the engaging part on the surface opposed to the regulation part is formed wider than that of the projections of the engaging part on the surface opposed to the case; and wherein an air conduction groove is provided in between each of the plurality of projections in the radial direction. a lens holder configured to hold an image pickup lens; and a case configured to have an insertion hole, in which the lens holder is inserted, formed in the case, wherein the lens holder is configured to include: an insertion part for being inserted into the insertion hole of the case in an insertion direction which is an optical axis direction of the image pickup lens, and a regulation part for regulating the insertion, wherein engaging parts (252, 352) are provided on a surface opposed to the case of the regulation part of the lens holder and a surface opposed to the regulation part of the case, and the engaging parts are formed so that the position of the lens holder relative to the case is adjustable in a direction perpendicular to the optical axis direction in a state where the engaging parts are engaged with each other; wherein the engaging part on the surface of the regulation part has a plurality of concentric ring-shaped projections formed therein surrounding the insertion part, and the engaging part on the surface of the case has a plurality of concentric ring-shaped projections formed therein surrounding the insertion hole, and then, the plurality of projections of the engaging part are inserted between the projections of the other engaging part so that the engaging parts are engaged with each other, and an interval of the projections of the engaging part on the surface opposed to the case is formed wider than that of the projections of the engaging part on the surface opposed to the regulation part, and an interval of the projections of the engaging part on the surface opposed to the regulation part is formed wider than that of the projections of the engaging part on the surface opposed to the case; and wherein an air conduction groove is provided in between each of the plurality of projections in the radial direction. an insertion part for being inserted into the insertion hole of the case in an insertion direction which is an optical axis direction of the image pickup lens, and a regulation part for regulating the insertion, wherein engaging parts (252, 352) are provided on a surface opposed to the case of the regulation part of the lens holder and a surface opposed to the regulation part of the case, and the engaging parts are formed so that the position of the lens holder relative to the case is adjustable in a direction perpendicular to the optical axis direction in a state where the engaging parts are engaged with each other; wherein the engaging part on the surface of the regulation part has a plurality of concentric ring-shaped projections formed therein surrounding the insertion part, and the engaging part on the surface of the case has a plurality of concentric ring-shaped projections formed therein surrounding the insertion hole, and then, the plurality of projections of the engaging part are inserted between the projections of the other engaging part so that the engaging parts are engaged with each other, and an interval of the projections of the engaging part on the surface opposed to the case is formed wider than that of the projections of the engaging part on the surface opposed to the regulation part, and an interval of the projections of the engaging part on the surface opposed to the regulation part is formed wider than that of the projections of the engaging part on the surface opposed to the case; and wherein an air conduction groove is provided in between each of the plurality of projections in the radial direction.