To provide an image forming spectroscope, which can coincide image forming positions of images in the vertical and literal directions with each other without generating astigmatism.

In an optical system 10, which is formed of a first and a second recessed surface mirrors M1, M2 arranged in a horizontal surface and a first diffraction grating G1, an image in the lateral direction of an intermediate slit S5 is formed in an out-going slit S6, and an image in the vertical direction is formed in rear side thereof at a distance (d). An aberration correcting optical system 11 is arranged in an incident side front stage of this optical system. The aberration correction optical system is provided with a third and a fourth concave mirrors M3, M4 arranged in a vertical surface, and an image in the lateral direction of an incident slit S3 is formed in the intermediate slit S5, and an image in the vertical direction is formed at a position K in front side thereof at a distance (d). Image forming displacement generated in the optical system 10 is off-set by generating an image forming displacement in the opposite direction in the aberration correcting optical system, and images in the vertical and lateral directions of the incident slit are formed on the out-going slit without generating astigmatism.