A substrate processing apparatus includes a processing chamber configured to process a substrate, a substrate support member provided within the processing chamber to support the substrate, a microwave generator provided outside the processing chamber, a waveguide launch port configured to supply a microwave generated by the microwave generator into the processing chamber, wherein the central position of the waveguide launch port is deviated from the central position of the substrate supported on the substrate support member and the waveguide launch port faces a portion of a front surface of the substrate supported on the substrate support member, and a control unit configured to change a relative position of the substrate support member in a horizontal direction with respect to the waveguide launch port.