An objective lens, comprising: in order from an object, a first lens group having positive refractive power as a whole, and a second lens group having negative refractive power as a whole, wherein the first lens group including: a positive lens which is disposed closest to the object and of which lens surface closest to the object is a plane or a concave surface; a diffractive optical element in which two diffractive element constituents made from different optical materials are cemented and which has a diffractive optical surface formed with diffraction grating grooves on the cemented surface; and at least one cemented lens (CL12, CL13, CL15), the second lens group consisting of two lens groups (CL21, CL22), wherein one of said two lens groups closer to the object side consists of a cemented meniscus lens having a convex surface facing toward the object side and concave surface facing toward an image, the other of said two lens groups including a concave surface facing the object, said concave surfaces of the second lens group facing each other, the following conditional expression is satisfied: 3 ≤ | f 2 / fa | ≤ 30 where fa denotes a focal length of the objective lens, and f2 denotes a focal length of the second lens group, the power Φ of a lens surface being defined by Φ = n ' − n / r where r denotes a radius of curvature of the lens surface, and n and n' are refractive indexes at the d-line of media before and after the lens surface, and the following conditional expression is satisfied: 1.03 ≤ | Φ 2 − 1 / Φ 2 − 2 | ≤ 1.4 where Φ2-1 denotes a power of the concave surface facing the image, and Φ2-2 denotes a power of the concave surface facing the object in the second lens group. in order from an object, a first lens group having positive refractive power as a whole, and a second lens group having negative refractive power as a whole, wherein the first lens group including: a positive lens which is disposed closest to the object and of which lens surface closest to the object is a plane or a concave surface; a diffractive optical element in which two diffractive element constituents made from different optical materials are cemented and which has a diffractive optical surface formed with diffraction grating grooves on the cemented surface; and at least one cemented lens (CL12, CL13, CL15), the second lens group consisting of two lens groups (CL21, CL22), wherein one of said two lens groups closer to the object side consists of a cemented meniscus lens having a convex surface facing toward the object side and concave surface facing toward an image, the other of said two lens groups including a concave surface facing the object, said concave surfaces of the second lens group facing each other, the following conditional expression is satisfied: 3 ≤ | f 2 / fa | ≤ 30 where fa denotes a focal length of the objective lens, and f2 denotes a focal length of the second lens group, the power Φ of a lens surface being defined by Φ = n ' − n / r where r denotes a radius of curvature of the lens surface, and n and n' are refractive indexes at the d-line of media before and after the lens surface, and the following conditional expression is satisfied: 1.03 ≤ | Φ 2 − 1 / Φ 2 − 2 | ≤ 1.4 where Φ2-1 denotes a power of the concave surface facing the image, and Φ2-2 denotes a power of the concave surface facing the object in the second lens group.