To provide a micro optical system which is adaptive to high specifications by increasing performance by effectively using a grating.

This system comprises two surfaces which are a plane (S1, S3) and a convex surface (S2, S4) in order from a long conjugation-length side. Luminous flux which is made incident from the 1st plane (S1) is reflected by the 2nd surface (S2), further reflected by the 3rd surface (S3) positioned in the center of the 1st surface (S1), and imaged nearby the surface peak of the 4th surface (S4) positioned nearby the 2nd surface (S2). The 2nd surface (S2) has a grating having aberration correcting action.