A tuner (43) includes a microwave transmission path (51), having a coaxial structure, configured to transmit a supplied microwave to a planar slot antenna (101); slugs (61a) and (61b made of a dielectric material and configured to be moved along the microwave transmission path (51); a slug driving unit (70) configured to move the slugs (61a) and (61b) along the microwave transmission path (51); and a control unit (80) configured to perform impedance matching by controlling positions of the slugs such that they are located at a matching position where a reflection coefficient is small, and configured to control, based on a state of the plasma, a matching track through which the slugs (61a) and (61b) reach the matching position.