A ground conductor (1) has a through hole provided through an area thereof for connection with a waveguide (6), with dimensions substantially equal to cavity dimensions of the waveguide (6), and a metallic spacer (7a) is provided as a holding element for a film substrate (4), with an even thickness to a dielectric substrate (2a), the metallic spacer (7a) having dimensions E1 and E2 of cavity walls thereof changed in accordance with a desirable frequency, and cooperating with another metallic spacer (7b) having substantially equal dimensions to the metallic spacer (7a), to sandwich the film substrate (4) in between, and in addition, an upper ground conductor (5) is arranged on the other metallic spacer (7b), and a quadrate resonant patch pattern (8) is formed at an end of the strip line conductor (3) formed to the film subs ate (4), on an area corresponding to a transducer end of the waveguide (6), while a combination of the quadrate resonant patch pattern (8) and the waveguide (6) is arranged such that the quadrate resonant patch pattern (8) has a center position thereof coincident with a center position of the cavity dimensions of the waveguide (6).