In certain types of microwave ovens, e.g. ovens of the single-wall type, the manufacturing of the oven cavity (10) involves the forming of at least one groove-­shaped recess (16) in at least one of the cavity walls (14). In order to substantially eliminate the disturbing effect of this recess (16) on the microwave field pattern in the oven cavity (10), a reflecting element in the shape of an elongated strip (18) of conductive material is provided in a position at the entrance opening of the recess (16), as seen from the oven cavity (10). The conductive strip (18) is preferably covered by dielectric material, e.g. in the shape of a dielectric plate (20) that also serves as support for the strip (18), and this strip (18) is so dimensioned and disposed at the opening of the recess (16) that it substantially reflects all incident microwave radiation coming from the oven cavity (10) at the opera­tional frequency of the microwave oven.