Autofocus adapter

An autofocus adapter (10) is provided and includes a first optical system (10A) placed on the optical axis (Z1) of an imaging lens (20) and a focus state detection adapter second optical system (10B) placed on an optical axis (Z2) different from that of the imaging lens. The imaging adapter optical system (10A) has a light split prism (P1) for splitting subject light passing through the imaging lens into imaging subject light and the focus state detection subject light, a first lens group (L1) having negative power and placed on the object side with respect to the light split prism, and a second lens group (L2) having a positive power as a whole and placed on the image side with respect to the light split prism.