A method of diagnosing a combustion state in a furnace of a boiler wherein the shape of the root part of a flame (namely, the part closer to the fuel jet port of a burner) is picked up by an image sensor or the like, and the combustion state is diagnosed from the shape. Concretely, the relationships between the shapes of the root parts of flames and the combustion states of the furnace, e.g., the amount of CO or the amount of NOx are experimentally or empirically ensured and are stored in advance, one of the patterns of the stored flame shapes is selected on the basis of the shape of the root part of the flame actually observed, and the combustion state of the furnace is judged the combustion state of the selected flame shape.