A combustion device for improving a turndown ratio, the device comprising: a premixing chamber connected to an air supply tube and a gas supply tube, the premixing chamber having an inner space in which air and gas for combustion are mixed with each other, wherein the inner space of the premixing chamber, in which the air and gas supplied through the air supply tube and the gas supply tube are mixed with each other is divided into a multiple-stage venturi structure, a mixed gas regulation unit that opens or closes a portion of the premixing chamber divided into the multiple-stages to regulate a flow rate of a mixed gas of the air and gas, wherein the premixing chamber is divided into two stages to form a first premixing chamber and a second premixing chamber in both sides of a partition member, and the mixed gas regulation unit opens or closes the second premixing chamber through which the air passes and into which the gas is discharged wherein the combustion device is adapted so that gas discharged into the premixing chamber through the gas supply tube is discharged in parallel to a flow direction of the air supplied into the premixing chamber through the air supply tube, and wherein the gas supply tube is branched into a first gas discharge tube supplying the gas into the first premixing chamber and a second gas discharge tube supplying the gas into the second premixing chamber, and wherein a first discharge hole of the first gas discharge tube and a second discharge hole of the second gas discharge tube are formed so that the gas is discharged toward outlets (312, 322) of the first premixing chamber and the second premixing chamber, and wherein the mixed gas regulation unit comprises a moving block reciprocated by a driving unit to open or close the second discharge hole of the second gas discharge tube and a flow path of the air passing through the second premixing chamber simultaneously. a premixing chamber connected to an air supply tube and a gas supply tube, the premixing chamber having an inner space in which air and gas for combustion are mixed with each other, wherein the inner space of the premixing chamber, in which the air and gas supplied through the air supply tube and the gas supply tube are mixed with each other is divided into a multiple-stage venturi structure, a mixed gas regulation unit that opens or closes a portion of the premixing chamber divided into the multiple-stages to regulate a flow rate of a mixed gas of the air and gas, wherein the premixing chamber is divided into two stages to form a first premixing chamber and a second premixing chamber in both sides of a partition member, and the mixed gas regulation unit opens or closes the second premixing chamber through which the air passes and into which the gas is discharged wherein the combustion device is adapted so that gas discharged into the premixing chamber through the gas supply tube is discharged in parallel to a flow direction of the air supplied into the premixing chamber through the air supply tube, and wherein the gas supply tube is branched into a first gas discharge tube supplying the gas into the first premixing chamber and a second gas discharge tube supplying the gas into the second premixing chamber, and wherein a first discharge hole of the first gas discharge tube and a second discharge hole of the second gas discharge tube are formed so that the gas is discharged toward outlets (312, 322) of the first premixing chamber and the second premixing chamber, and wherein the mixed gas regulation unit comprises a moving block reciprocated by a driving unit to open or close the second discharge hole of the second gas discharge tube and a flow path of the air passing through the second premixing chamber simultaneously.