An object of the present invention is to reduce the diameter, to make focus adjustment easy, and to achieve bright, wide-angle, high-resolution image quality.Provided is an endoscope objective optical system including, in this order from the object side, a front group having negative refractive power, an aperture stop, and a positive rear group. The front group includes, in this order from the object side, a first lens, which is a single lens having negative refractive power, and a second lens, which is a single lens having positive refractive power. The rear group includes a third lens, which is a single lens having positive refractive power, a cemented lens formed of a fourth lens having positive refractive power and a fifth lens having negative refractive power, and a sixth lens having positive refractive power. The object-side surface of the first lens is a flat surface, the second lens has as meniscus shape, and the sixth lens is joined to the image-acquisition element, and the endoscope objective optical system satisfies the following conditional expression:4
