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 (14a) and rear walls (14b)(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.