A combustion device with high combustion efficiency comprising: a body; and a burner tube arranged in the body, wherein the burner tube extends lengthwise on a plane and has a mixing section in which gas and air are mixed and an outlet section in which a combustible mixture the gas and the air is discharged along its length, wherein the mixing section has a first air inlet for air at a first end thereof and extends lengthwise along a reference line, wherein the mixing section includes a second air inlet for admitting air extending therethrough in a direction transverse to the reference line, wherein the mixing section has a first side and a second side on opposite sides and the second side is adjacent to a Y-axis of a coordinate system, wherein the second air inlet extends on the second side, wherein the mixing section forms a structure of a venturi tube configured to accelerate the flow speed of the combustible mixture of the gas and the air, wherein the structure of the venturi tube extends lengthwise along the reference line, wherein the structure of the venturi tube has a first end, a second end, and a middle between the first and the second ends, wherein the structure of the venturi tube has a decreasing width as it extends lengthwise from the first end to the middle and an increasing width as it extends lengthwise from the middle to the second end, wherein the mixing section forms a structure of a chamber which delimits the first air inlet and the second air inlet is disposed on the structure of the chamber, wherein the outlet section has a first straight part and a first curved part along its length sequentially, wherein the first straight part extends axially along the reference line, and wherein the first straight part extends between the mixing section and the first curved part; wherein since the opening direction of the second air inlet and the bending direction of the first curved part face the same side of the mixing section, the combustible mixture of the gas and the air can smoothly flow in the first curved part; and wherein since the air flowing through the second air inlet into the mixing section flows transversely to the gas, the speed of the combustible mixture of the gas and the air in the burner tube is not greatly lowered; and wherein the first curved part can reduce the speed loss of the combustible mixture of the air and the gas flowing in the burner tube, and can prevent the initial velocity of the gas in the mixing section from being degraded whereby the mixing section can introduce more air to improve combustion efficiency. a body; and a burner tube arranged in the body, wherein the burner tube extends lengthwise on a plane and has a mixing section in which gas and air are mixed and an outlet section in which a combustible mixture the gas and the air is discharged along its length, wherein the mixing section has a first air inlet for air at a first end thereof and extends lengthwise along a reference line, wherein the mixing section includes a second air inlet for admitting air extending therethrough in a direction transverse to the reference line, wherein the mixing section has a first side and a second side on opposite sides and the second side is adjacent to a Y-axis of a coordinate system, wherein the second air inlet extends on the second side, wherein the mixing section forms a structure of a venturi tube configured to accelerate the flow speed of the combustible mixture of the gas and the air, wherein the structure of the venturi tube extends lengthwise along the reference line, wherein the structure of the venturi tube has a first end, a second end, and a middle between the first and the second ends, wherein the structure of the venturi tube has a decreasing width as it extends lengthwise from the first end to the middle and an increasing width as it extends lengthwise from the middle to the second end, wherein the mixing section forms a structure of a chamber which delimits the first air inlet and the second air inlet is disposed on the structure of the chamber, wherein the outlet section has a first straight part and a first curved part along its length sequentially, wherein the first straight part extends axially along the reference line, and wherein the first straight part extends between the mixing section and the first curved part; wherein since the opening direction of the second air inlet and the bending direction of the first curved part face the same side of the mixing section, the combustible mixture of the gas and the air can smoothly flow in the first curved part; and wherein since the air flowing through the second air inlet into the mixing section flows transversely to the gas, the speed of the combustible mixture of the gas and the air in the burner tube is not greatly lowered; and wherein the first curved part can reduce the speed loss of the combustible mixture of the air and the gas flowing in the burner tube, and can prevent the initial velocity of the gas in the mixing section from being degraded whereby the mixing section can introduce more air to improve combustion efficiency.