A row of rich-side flame holes is centrally arranged. Two rows of lean-side flame holes are arranged on both sides of the rich-side flame hole row, respectively. In addition, two rows of rich-side flame holes are arranged on the outsides of the two lean-side flame hole rows, respectively. A lower end part of a central rich-side burner part is projected into a tubular part into which the rich-side mixture is introduced, and communication holes in fluid communication with an inner space are formed in walls on both sides so as to pass completely therethrough in alignment with each other. Each communication hole has a larger diameter than an inner width P and is disposed at a potion situated nearer to the upper of the tubular part and nearer to the front so as to leave, at the rear, a space in which dust p particles are accumulated.