The present invention provides a biomass combustion burner applied to a pulverized coal-fired boiler to burn biomass fuel, a biomass-mixed fired boiler that reduces an amount of CO2 derived from fossil fuels, and a method for burning biomass fuel using the foregoing. The biomass combustion burner includes a biomass fuel jet nozzle having a fuel jet port that jets biomass fuel conveyed by primary air; a secondary air nozzle having a secondary air jet port that surrounds the fuel jet port; and a tertiary air nozzle having a tertiary air jet port that surrounds the secondary air jet port. The biomass fuel jet nozzle includes a fuel concentration adjusting section that changes a biomass fuel stream into a swirl flow to thereby make a fuel concentration higher on an outer circumferential portion side; and a degree-of-swirl adjusting plate that reduces a degree of swirl of a jetting fuel stream.
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