An antenna, a dielectric window, a plasma processing apparatus and a plasma processing method are capable of improving uniformity of a substrate surface processing amount in the surface of the substrate. The antenna includes the dielectric window 16; and a slot plate 20, provided on one side of the dielectric window 16, having a plurality of slots 133. The dielectric window 16 has a flat surface 146 surrounded by a ring-shaped first recess; and a plurality of second recesses 153 formed on the flat surface 146 so as to surround a center of the flat surface 146. Here, the flat surface 146 is formed on the other side of the dielectric window 16. When viewed from a thickness direction of the slot plate, a center of each second recess 153 is located within each slot 133 of the slot plate.