PROBLEM TO BE SOLVED: To provide an optical imaging system which offers a short total length and high imaging quality.SOLUTION: An optical imaging system includes a main objective lens 20 and a reduction optical unit. The reduction optical unit comprises a first lens 22 having positive refractive power and a second lens 21 having negative refractive power. The first lens 22 is made from a first material having a first Abbe number and the second lens 21 is made from a second material having a second Abbe number, where the first Abbe number is greater than the second Abbe number. The lens system satisfies the following relational expression for a wavelength range (&lambda；) of 480 nm&le；&lambda；&le；660 nm and for a main wavelength e=546 nm. In the relational expression, frepresents an object-side focal length for the main wavelength (e) with respect to a first main plane (H), and frepresents an object-side focal length for the wavelength (&lambda；) with respect to the first main plane (H), while f' represents an object-side focal length for the main wavelength (e) with respect to a second main plane (H').【課題】全長が短く、高い撮像品質を持つ光学撮像システムの提供。【解決手段】主対物レンズ２０と、縮小光学ユニットとを備え、縮小光学ユニットは正の屈折力を有する第１のレンズ２２、及び負の屈折力を有する第２のレンズ２１からなり、第１のレンズ２２は第１のアッベ数を有する第１の材料から製造され、第２のレンズ２１は第２のアッベ数を有する第２の材料から製造され、第１のアッベ数は第２のアッベ数より大きい。レンズ系は、波長λの範囲４８０ｎｍ≦λ≦６６０ｎｍ及び主波長ｅ＝５４６ｎｍに関して以下の関係式を満たす。ｆe＝第１の主平面Ｈに対する、主波長ｅに関する対象側焦点距離；ｆλ＝第１の主平面Ｈに対する、波長λに関する対象側焦点距離；ｆe‘＝第２の主平面Ｈ‘に対する、主波長ｅに関する対象側焦点距離【選択図】図１
