A sample holder unit includes a sample holding portion, a shielding plate, and a rotating mechanism. The rotating mechanism supports the sample and the shielding plate and allows the sample and the shielding plate to rotate. The rotating mechanism is oriented such that an axis of rotation thereof is parallel to both a processing surface of the sample and a direction orthogonal to a direction of projection of the sample from the shielding plate. A center of rotation of the rotating mechanism resides on the processing surface of the sample.