1. An image forming system, comprising: a data conversion unit that thins out first image data configured by arranging dot data in matrix fashion and converts two-line data of the first image data into one-line data of second image data, the dot data representing either one of image forming dot data with which an image is to be formed and non-image forming dot data with which an image is to be not formed; and an image output unit that outputs an image obtained by reproducing the second image data converted by the data conversion unit, wherein the data conversion unit includes: a determination unit which divides the first image data into a dot block each of which includes first and second dot data in a first row and third and fourth dot data in a second row in a first order from the first dot data to the second dot data, and determines whether or not an array of dot data included in each dot block corresponds to a first array, a second array or a third array, wherein the first array includes the first and fourth dot data each representing the non-image forming dot data, and the second and third dot data, at least one of which represents the image forming dot data, wherein the second array includes the first and fourth data, each representing the image forming dot data, and the second and third dot data each representing the non-image forming dot data, and wherein the third array includes the first to third dot date each representing the image forming dot data, and the fourth dot data representing the non-image forming dot data or includes the first dot data representing the non-image forming dot data and the second to fourth dot data representing the image forming dot data; and an output unit which, when the determination unit determines that the array in a dot block corresponds to the first array, outputs, as a part of one line data of the second image data, dot data including the image forming dot data and the non-image forming dot data, when the determination unit determines that the array in a dot block corresponds to the second array, outputs, as a part of one line data of the second image data, dot data including the image forming dot data and the non-image forming dot data, when the determination unit determines that the array in a dot block corresponds to the third array, outputs, as a part of one line data of the second image data, dot data including two of the image forming dot data, and when the determination unit determines that the array in a dot block does not correspond to any of the first to third arrays, outputs as a part of one line data of the second image data, dot data including the dot data represented by the first dot data and the dot data represented by the fourth dot data in the first order.