The invention provides a boiler provided with low cost means which can reduce a concentration of CO, an unburned matter, an attached ash and the like near a side wall and maintain a combustion state well with a simple structure. In a boiler having a combustion chamber 13 formed by front and rear walls (burner walls) provided with a plural stages of burners 2, 3 and 4 on at least one of them and opposing to each other, and side walls 1a and 1b crossing to said burner walls 14a and 14b, a gas port 6 containing no fuel for making a pressure of a gas near said side walls 1a and 1b within said combustion chamber 13 higher than a pressure of a gas at a center portion of said combustion chamber 13 is provided between an outermost burner and the side walls 1a and 1b within a range of a height of said burner stages 2, 3 and 4. A combustion gas 16 can not come close to the side walls 1a and 1b due to the jet 18 of the gas.