In a premixing combustion device (1) for a heating boiler, comprising a blower (2) which delivers a combustion mixture composed of fuel and air from an outlet opening (8) of the blower (2) via a burner cylinder (9) to a combustion zone, wherein a mixing device (3) arranged upstream of the blower (2) prepares the combustion mixture, it is sought to provide a solution which, in a structurally simple manner, ensures reliable starting of the heating boiler and prevents failures. This is achieved by means of a throughflow reduction device (10) which is arranged between the outlet opening (8) of the blower (2) and the burner cylinder (9) and which has a throughflow reduction element (13) which can be adjusted between a position in which it opens the outlet opening (8) of the blower (2) and a position in which it reduces the size of the outlet opening (8) of the blower (2).