In the lens assembly, ventilation channels 15b1, 15c1, 15d1, and 15e1 passing between an object-side opening 15.sub.--1 and an image-side opening 15.sub.--2 of a barrel 15 are formed by processing inner surfaces 15b, 15c, and 15d of the barrel 15 and a surface 15e of the barrel 15. Thereby, when a taking lens 11 of a first group having an O-ring 16 fitted thereto is mounted into the barrel 15, air in the barrel 15 is pushed out from the image-side opening 15.sub.--2 of the barrel 15 through the ventilation channels 15b1, 15c1, 15d1, and 15e1.