Abstract: An apparatus for continuously concentrating and denitrating a radioactive nitrate solution by microwave includes a rotatably supported concentrating and denitrating vessel disposed within an oven into which microwave is applied through waveguides. The concentrating and denitrating vessel comprises a circular vessel body supported rotatably about its center and a plurality of partition plates positioned radially inside the vessel body so as to divide the interior of the vessel body into a plurality of compartments. The nitrate solution supplied into the vessel is heated, evaporated, concentrated and denitrated in sequence in each of the compartments during one rotation of the vessel to form a denitrated oxide powder. The denitrated powder produced in one of the compartments is transferred into a receiving pan positioned adjacent to the outer periphery of the vessel.