By reducing the outer diameter, increasing the overall length, and correcting chromatic aberration well, the invention realizes an objective optical system and examination apparatus that are suitable for in-vivo examination with a sufficiently increased length and widened observation range. The invention provides a compact objective optical system and an examination apparatus comprising, in order from an object side: a first lens group having positive power; a second lens group including a compound lens; a third lens group formed of a biconvex lens; a fourth lens group having positive power and including a compound lens; and a fifth lens group having positive power. Joining surfaces of the compound lens included in the second lens group and the compound lens included in the fourth lens group have negative power.