An adaptor is attachable between an interchangeable lens including a first light amount adjustment unit capable of changing a passing area of image pickup light, and a camera main body including an image pickup element. The adaptor includes: a second light amount adjustment unit capable of changing transmittance of the image pickup light by using a change in a physical property; and a controller controlling the second light amount adjustment unit. In image pickup mode, the controller controls the transmittance in the second light amount adjustment unit depending on an operating amount of an operation unit by a user to adjust a light amount, and in the image pickup mode, the controller converts an instruction, transmitted toward the interchangeable lens from the camera main body, to drive the first light amount adjustment unit, into a drive completion signal, and transmits the drive completion signal to the camera main body.