A combustor, comprising: two upstream parts (201, 201a; 202, 202a) of a fuel supply system for supplying gaseous fuels of two types (201; 202) having different heating values from each other; a three-way fuel transfer valve for merging the two upstream parts (201, 201a; 202, 202a) of the fuel supply system with each other; a plurality of gaseous fuel supply subsystems for supplying a combustion chamber with the gaseous fuels supplied through the three-way fuel transfer valve; a plurality of burners for injecting, corresponding to each of the gaseous fuel supply subsystems (210a-210e), the gaseous fuel supplied from the gaseous fuel supply subsystem into the combustion chamber; characterized in that a low Btu gas-exclusive subsystem disposed in parallel with the gaseous fuel supply subsystems (210a-210e); and a burner for injecting the low Btu gas supplied from the low Btu gas-exclusive subsystem into the combustion chamber, wherein: the low Btu gas-exclusive subsystem includes: a gaseous fuel flow control valve for adjusting the flow rate of the low Btu gas; a gas shutoff valve, disposed upstream of the gaseous fuel flow control valve, for preventing a high Btu gas from leaking to the low Btu gas-exclusive subsystem during the single fuel combustion of the high Btu gas; and a purge subsystem disposed downstream of the gaseous fuel flow control valve. two upstream parts (201, 201a; 202, 202a) of a fuel supply system for supplying gaseous fuels of two types (201; 202) having different heating values from each other; a three-way fuel transfer valve for merging the two upstream parts (201, 201a; 202, 202a) of the fuel supply system with each other; a plurality of gaseous fuel supply subsystems for supplying a combustion chamber with the gaseous fuels supplied through the three-way fuel transfer valve; a plurality of burners for injecting, corresponding to each of the gaseous fuel supply subsystems (210a-210e), the gaseous fuel supplied from the gaseous fuel supply subsystem into the combustion chamber; characterized in that a low Btu gas-exclusive subsystem disposed in parallel with the gaseous fuel supply subsystems (210a-210e); and a burner for injecting the low Btu gas supplied from the low Btu gas-exclusive subsystem into the combustion chamber, wherein: the low Btu gas-exclusive subsystem includes: a gaseous fuel flow control valve for adjusting the flow rate of the low Btu gas; a gas shutoff valve, disposed upstream of the gaseous fuel flow control valve, for preventing a high Btu gas from leaking to the low Btu gas-exclusive subsystem during the single fuel combustion of the high Btu gas; and a purge subsystem disposed downstream of the gaseous fuel flow control valve. the low Btu gas-exclusive subsystem includes: a gaseous fuel flow control valve for adjusting the flow rate of the low Btu gas; a gas shutoff valve, disposed upstream of the gaseous fuel flow control valve, for preventing a high Btu gas from leaking to the low Btu gas-exclusive subsystem during the single fuel combustion of the high Btu gas; and a purge subsystem disposed downstream of the gaseous fuel flow control valve. a gaseous fuel flow control valve for adjusting the flow rate of the low Btu gas; a gas shutoff valve, disposed upstream of the gaseous fuel flow control valve, for preventing a high Btu gas from leaking to the low Btu gas-exclusive subsystem during the single fuel combustion of the high Btu gas; and a purge subsystem disposed downstream of the gaseous fuel flow control valve.