The present invention discloses an image-forming optical system provided with a plurality of curved mirrors whereby two points at different distances are made to have an optically conjugate relationship, sequentially starting with a first conjugate point which is nearer when an optical path is traced from the first conjugate point to a second conjugate point which is farther, comprises, a first mirror which reflects luminous flux from the first conjugate point to transform the luminous flux into substantially parallel luminous flux, and a second mirror which reflects the luminous flux reflected by the first mirror while keeping the luminous flux substantially parallel. Further, the optical system satisfies the following condition:

|Arctan(1/2F)-|2(-)||10[deg]