A mixing device for mixing a first substance and a second substance to form a mixed substance; wherein said mixing device comprises a housing having a first inlet for said first substance, a second inlet for said second substance and an outlet for said mixed substance; wherein a flow channel is arranged between said first inlet and said outlet, said flow channel having a flow axis between said first inlet and said outlet, and wherein said flow channel is provided with a constricted Venturi tube section, said Venturi tube section having a side wall; wherein a lateral feed opening is provided in said side wall in a first lateral direction, which lateral feed opening is connected to said second inlet, and said opening having a feed axis in said first lateral direction which is substantially perpendicular to the flow axis of said flow channel; wherein upstream from said lateral feed opening the side wall of said Venturi tube section forms a first constricted part having a first width in said first lateral direction and a second width in a second lateral direction perpendicular to said first lateral direction; wherein said second width is larger than said first width; characterized in that downstream from said lateral feed opening the side wall of said Venturi tube section forms a second constricted part having a third width in said first lateral direction and a fourth width in said second lateral direction, wherein seen in said first lateral direction, on a first lateral side of the flow channel the wall of said second constricted part is substantially flush with the wall of said first constricted part, and the flow channel is widened in said second constricted part in the direction of a second lateral side of the flow channel such that said third width is larger than said first width, said second lateral side being opposite the first lateral side, wherein the flow channel is widened in said second constricted part at the side of the lateral feed opening, wherein only the third width of the flow channel of the second constricted part is widened at the side of the lateral feed opening. wherein said mixing device comprises a housing having a first inlet for said first substance, a second inlet for said second substance and an outlet for said mixed substance; wherein a flow channel is arranged between said first inlet and said outlet, said flow channel having a flow axis between said first inlet and said outlet, and wherein said flow channel is provided with a constricted Venturi tube section, said Venturi tube section having a side wall; wherein a lateral feed opening is provided in said side wall in a first lateral direction, which lateral feed opening is connected to said second inlet, and said opening having a feed axis in said first lateral direction which is substantially perpendicular to the flow axis of said flow channel; wherein upstream from said lateral feed opening the side wall of said Venturi tube section forms a first constricted part having a first width in said first lateral direction and a second width in a second lateral direction perpendicular to said first lateral direction; wherein said second width is larger than said first width; characterized in that downstream from said lateral feed opening the side wall of said Venturi tube section forms a second constricted part having a third width in said first lateral direction and a fourth width in said second lateral direction, wherein seen in said first lateral direction, on a first lateral side of the flow channel the wall of said second constricted part is substantially flush with the wall of said first constricted part, and the flow channel is widened in said second constricted part in the direction of a second lateral side of the flow channel such that said third width is larger than said first width, said second lateral side being opposite the first lateral side, wherein the flow channel is widened in said second constricted part at the side of the lateral feed opening, wherein only the third width of the flow channel of the second constricted part is widened at the side of the lateral feed opening. wherein seen in said first lateral direction, on a first lateral side of the flow channel the wall of said second constricted part is substantially flush with the wall of said first constricted part, and the flow channel is widened in said second constricted part in the direction of a second lateral side of the flow channel such that said third width is larger than said first width, said second lateral side being opposite the first lateral side, wherein the flow channel is widened in said second constricted part at the side of the lateral feed opening, wherein only the third width of the flow channel of the second constricted part is widened at the side of the lateral feed opening.