The endoscope optical system consists of: a first lens group that is used only for front viewing; a plurality of second lens groups that have a negative lens at a position closest to an object side and are used only for side viewing; a third lens group that is commonly used in front viewing and side viewing; and a synthesizing section that synthesizes the rays emitted from the first lens group and the rays emitted from the second lens groups and causes the synthesized rays to be incident into the third lens group. The rays emitted from the first lens group and the rays emitted from the second lens groups are imaged on a same plane.