The resonance chamber (1) has the form of an elongated closed shape. Microwave generators are fastened to the walls (1.1,1.2) of the chamber (1), their antennae (3,4) directed to the inside of the chamber (1). The microwave generators with antennae (3) are fastened to the chamber wall (1.1), while the microwave generators with antennae (4) are fastened to the wall (1.2) of the resonance chamber. The microwave generators on the wall (1.1) of the chamber are fastened so that in an orthogonal projection into the opposite wall of the chamber the antennae of two neighboring magnetrons are situated on the opposite sides of the product flow conduit (2). The antennae (3) of the successive magnetrons of the first series are situated alternately on the left and on the right side of the product flow conduit (2). Similarly distributed are the antennae (4) of the successive magnetrons of the second row.