Abstract: One particle group in a sample undergoing measurement is demarcated from another particle group in the sample by drawing a discriminant on a plot of a two-dimensional distribution of the particles. To draw the discriminant, the two-dimensional distribution is divided into a grid pattern of a plurality of areas arranged in a number of columns, the number of particles contained in each area is determined, and an area containing the smallest number of particles is selected from a plurality of areas in a specific column. Next, in a column neighboring the specific column, an area containing the smallest number of particles is selected from those areas adjacent the area selected in the specific column. This operation for the neighboring column is thenceforth successively repeated for all subsequent columns to select the area containing the smallest number of particles in each column.