An oligomerization process for the production of higher aliphatic olefins is disclosed. A mass portion partly overlapping the supporting portion, as viewed in a plan view, is arranged on at least one of two main surfaces of the movable portion. An end of the first waveguide is directly or indirectly connected so as to be conductive to one of ground electrodes of the second waveguide from the direction orthogonal to the stacking direction of the ground electrodes.