A steam generator is operated under both of supercritical pressure and subcritical pressure and has having generating tubes that form a furnace wall. Upper and lower generating tubes which are directed vertically and central generating tubes are inclined by 10.degree. to 35.degree. with respect to a vertical line. The steam generator further includes a burner wind box which is inclined along the inclination of the central generating tubes and are vertically divided into a plurality of stages.