There is provided a bright objective optical system with a high performance, suitable for a small-size image sensor with a large number of pixels, in which the focusing in accordance with a change in the focal length is possible and manufacturing error sensitivity is lowered. The objective optical system includes in order from an object side, a first group G1 having a positive refractive power, a second group G2 having a negative refractive power, and a third group G3 having a positive refractive power, wherein focusing is carried out by moving the second group G2, and a lens nearest to image is a planoconvex positive lens L8 having a convex surface directed toward the object side, and a flat surface of the planoconvex positive lens L8 is either affixed directly to an image pickup surface I or cemented to a cover glass CG formed on the image pickup surface I, and the objective optical system satisfies the following conditional expression (1). 5 <； ff / f <； 8 where, ff denotes a focal length of the planoconvex positive lens disposed nearest to image, and f denotes a focal length of the overall objective optical system at the time of normal observation.
