To provide a component mounting apparatus automatically mounting an elastically deformable ring-shaped component such as a gasket in the right condition without being twisted with respect to a mounting portion.

A distal end portion of an extruding plate 53 abuts against the upper surface of the gasket 111 elastically deformed and enlarged in diameter, and the extruding plate 53 pushes down the gasket 111. The extruding plate 53 further pushes down the gasket 111 to allow a projected portion 107 of a body 103 to pass through. Hereat, the inner surface of the gasket 111 has the enlarged-diameter of a dimension to avoid contact with the outer surface of the projected portion 107 of a lamp socket 101, and therefore the gasket 111 passes through the projected portion 107 with little or no contact with the outer surface of the projected portion 107. After passing through the projected portion 107, the gasket 111 is reduced in diameter by its own elastic force and then mounted on the mounting portion 109. The gasket 111 is not twisted because the gasket 111 passes through the projected portion 107 with little or no contact with the outer surface of the projected portion 107.