A second lens (15) of a concave meniscus type is formed from a plastic nanocomposite material. An approximately annular flange (15b) is formed along an outer periphery of a lens body portion (15a). The lens body portion (15a) and the flange (15b) are formed to satisfy 1 < (Lt/Ft) < 5 and (CA/4) = b. 'CA' is a diameter of the lens body portion (15a). 'Ft' is a thickness at a center of the lens body portion (15a). 'Lt' is a thickness of the flange (15b) in an optical axis direction. 'R' is an outer diameter of the flange (15b). 'b' is one-half of a difference between the outer diameter R and the diameter CA. Increasing the thickness of the flange (15b) increases mechanical strength of the second lens (15), thus preventing the second lens (15) from being damaged easily.