Optical system and plane spectroscopic device

An optical system to divide a light flux from an object plane, the optical system comprising:

• a first curved-surface mirror having an opening through which the light flux from the object plane passes or a transmitting portion through which the light flux is transmitted;
• a second reflecting portion including a plurality of reflective surfaces configured to divide the light flux from the opening or the transmitting portion of the first curved-surface mirror to result in second light fluxes, wherein the second reflecting portion is configured to guide each of the second light fluxes to respective positions, different from one another, on the first curved-surface mirror;
• a third reflecting portion having a plurality of reflective surfaces, each of which reflects, as third light fluxes, first light fluxes that have been reflected on the first curved-surface mirror;
• and
• a fourth reflecting portion having a plurality of reflective surfaces configured to reflect the third light fluxes from the third reflecting portion,
• wherein a number of reflective surfaces of each of the third reflecting portion and the fourth reflecting portion on which the respective first and third light fluxes are incident is the same as a division number in the dividing of the light flux into the second light fluxes by the second reflecting portion, and
• wherein the first and third light fluxes are reflected by the respective third and fourth reflecting portions to be image-formed so that divided images of the object plane are formed.