Attachment optical system and projection display system

An attachment optical system to detachably be attached to a magnification side of a projection optical system provided to a projection display device, and configured to project projection light emitted from the projection optical system on an imaging plane different from a magnification side imaging plane of the projection optical system, the attachment optical system comprising:

• an optical element having a second optical axis arranged on an extension of a first optical axis of the projection optical system, wherein:
• the optical element has:
• a plane of incidence arranged on the second optical axis, wherein the projection light emitted from the projection optical system is incident on the plane of incidence to proceed to a first reflecting surface,
• the first reflecting surface configured to reflect light emitted from the plane of incidence,
• a second reflecting surface configured to reflect light reflected by the first reflecting surface, and
• an exit surface configured to transmit light reflected by the second reflecting surface such that light passing through the exit surface proceeds directly to the imaging plane without passing through another optical element,
• the first reflecting surface and the exit surface are continuous in an axial area where light passes the second optical axis and the first optical axis, and
• the imaging plane of the attachment optical system is provided with a shape of an inside surface of a spherical portion obtained by dividing a sphere.