An objective lens (1) according to the invention includes a front-group lens G F , a rear-group lens G R which is arranged on the optical axis of the front-group lens G F and on a side opposite to an object plane (2) with the front-group lens G F interposed there between, and a semitransparent mirror (11) which is arranged between the front-goup lens G F and the rear-group lens G R , which reflects illumination into the front-group lens G F , and which transmits reflected light from the object plane (2) and enters the transmitted light into the rear-group lens G R . The front-group lens G F is made of a cemented lens which is configured by sticking two lenses L 4 and L 5 together.