To excellently compensate aberrations over a whole range with simple constitution by providing a moving lens group for compensating aberration caused by the change of the thickness of a cover glass, etc.

In an objective lens composed of a first to a third lens groups in order from the object side, the first lens group G1 has a positive refractive power for converging a light beam from an object, the second lens group G2 is composed of three joined lenses of positive, negative and positive lenses in order from the object side and having a small refractive power and the third lens group G3 has a negative refractive power. Compensation of aberration is performed by relatively moving the second lens group according to the thickness of a cover glass, etc. The respective lens groups satisfy the following conditional relations: f2/f>50, 1.1<(E1/E2)<1.3. where, f2: the focal distance of the second lens group, f: the focal distance of the whole system of objective lens, E1: maximum effective diameter of the first lens group, E2: maximum effective diameter of the second lens group.