A burner is arranged axially of a burner throat on a furnace wall and includes a nozzle body housed in a wind box and with a secondary air adjuster on a leading end of the nozzle body. The adjuster includes an end plate for defining together with a near-furnace side surface of the wind box a cylindrical space opened in an outer circumference thereof, a slide damper axially slidable for surrounding the cylindrical space, air vanes arranged at predetermined intervals and circumferentially of the cylindrical space for swirling a secondary air and drive means and for slide movement of the slide damper.