An antenna according to an aspect includes: a dielectric window having a first surface and a second surface, the second surface having an annular recessed surface and a flat surface surrounded by the recessed surface; a slot plate; a dielectric plate; a heat transfer member made of metal and having an upper surface and a lower surface opposing each other; a cooling jacket; and a heater, in which the upper surface includes a plurality of first regions and a second region, the cooling jacket is mounted on the plurality of first regions, the second region is recessed further toward the lower surface side than the plurality of first regions, the heater is mounted on the second region, and each of the plurality of first regions is provided at a position at least partially overlapping with the flat surface when viewed in a direction parallel to a central axis.