An imaging apparatus having an opening of a front casing caulked with a pressure member. An O-ring is used for caulking the front casing and is designed to have an inner diameter larger than the outer diameter of the first lens, and an outer diameter smaller than the inner diameter of the pressure member. When the pressure member is screw fitted, the O-ring is compressed. Thus, a clearance formed by the edge portion of the first lens and the inner wall surface of the pressure member is caulked by the deformation of the O-ring occurring when the O-ring is pressed.