A dielectric window for a plasma treatment device for a plasma treatment device that uses microwaves as a plasma source. The dielectric window is circular-plate-shaped and allows microwaves to propagate. The dielectric window has a recess that has an opening on the lower-surface side and that indents in the plate thickness direction of the dielectric window, and is provided to the lower surface at which plasma is generated when the dielectric window is provided to the plasma treatment device. The recess has a bottom surface extending in the direction perpendicular to the plate thickness direction, and a side surface extending in the plate thickness direction from the circumferential edge of the bottom surface toward the opening of the recess. In addition, an inclined surface extends at an incline relative to the plate thickness direction from the opening-side circumferential edge of the side surface toward the opening of the recess.