To provide a floating follow-up optical system with simple mechanism which suppresses displacement between optical axes within a tolerance and restrain chromatic aberration from generating, by optically correcting at least one of parallel moving components or gradient components between two optical axes even if mechanical displacement between optical axes is generated by relative displacement of plural optical systems.

This system consists of a first afocal optical system AFC1 belonging to a floating system and a second afocal optical system AFC2 belonging to a fixed system. The first afocal optical system AFC1 has first and second thin lenses L1, L2, and the second afocal system AFC2 has third and forth thin lenses L3, L4. The first and second afocal optical systems AFC1, AFC2 are designed to satisfy gradient follow-up conditions.