There is provided a high-performance endoscope objective optical system which is small-sized and is capable of securing an adequate observation depth, and which has a wide angle of view, and which makes easy an observation and diagnosis of a pathological lesion.The endoscope objective optical system consists of, in order from an object side, a front group having a negative refractive power, an aperture stop, and a rear group having a positive refractive power. The front group includes a first lens which is a single lens having a negative refractive power, and a second lens which is a single lens having a positive refractive power, in order from the object side, and the rear group includes a third lens which is a single lens having a positive refractive power, a cemented lens of a fourth lens having a positive refractive power and a fifth lens having a negative refractive power, and a sixth lens having a positive refractive power. An object-side surface of the first lens is a flat surface, and the second lens has a meniscus shape with a convex surface directed toward an image side, and the sixth lens is cemented to an image pickup element, and the following conditional expression (1) is satisfied:where,D6 denotes a thickness of the sixth lens, andF12 denotes a combined focal length from the first lens up to the second lens.
