Viewfinder unit with line-of-sight detection function, image capturing apparatus, and attachment accessory

A viewfinder unit includes an optical member between a display and an eyepiece, a first member holding the optical member in a direction orthogonal to an optical axis of the optical member, an illumination device, a second member holding the illumination device, a sensor capable of picking up a photographer's eye image, and an opening wider than a display area of the display in an optical axis direction of the optical member. When the eyepiece is viewed in the optical axis direction, the illumination device is inside the opening and outside the display area. Part of a first area of the optical member, projecting from an outline of the first member in the optical axis direction, is inside the opening in the orthogonal direction, and at least one of the illumination device and the second member is in a second area surrounded by the outline and the first area.