An imaging optical system is a system of a liquid immersion type permitting a plane-parallel plate in liquid to be replaced with another, without substantial degradation of imaging performance, while ensuring sufficiently high laser resistance of a boundary lens. The imaging optical system is provided with a first optically transparent member located nearest to a second plane, and a second optically transparent member located adjacent to the first optically transparent member. An optical path between the first optically transparent member and the second plane is fillable with a first liquid and an optical path between the first optically transparent member and the second optically transparent member is fillable with a second liquid. The imaging optical system satisfies the condition of 1