A method for producing water absorbent resin powder whose mass average particle diameter (D50), as defined by sieve classification and read by sieving 10 g of the water absorbent resin powder using JIS standard sieves specified in JIS Z8801-1 , classifying the sieved water absorbent resin powder using a sieve shaker (IIDA Sieve shaker, type ES-65) for 10 minutes at a relative humidity of 50% and at a temperature of 20°C or more and 25°C or less and plotting a residual percentage R on a logarithmic probability paper, is 300 μm or more and 10 mm or less and which contains less than 10 % by mass of fine powder having a particle diameter of 150 μm or less and has a surface cross-linked structure, said method being characterized by comprising: a polymerization step in which an unsaturated monomer aqueous solution is polymerized; a drying step in which a hydrogel cross-linked polymer obtained in the polymerization step is dried; a surface treatment step in which the hydrogel cross-linked polymer or a dried hydrogel cross-linked polymer is subjected to a surface treatment; and a sorting step, carried out after the drying step, in which black or brown particles of foreign matters dotted in a white water absorbent resin powder having a yellow index (YI) value of ≤ 20 are removed from the white water absorbent resin powder entirely or partially based on their colors using a sensor, wherein the sensor determines whether or not there is any coloring by using the color (bright) of a non-defective product sample as a background color, setting a signal at the time when the sensor monitors the background color as a standard and determining a certain standard signal range, comparing the color of a defective product sample and the background color with each other, and rating a signal deviating from the standard signal range as a color difference, and wherein an air jet is emitted and a digital image process is carried out so as to color-sort the foreign matters. a polymerization step in which an unsaturated monomer aqueous solution is polymerized; a drying step in which a hydrogel cross-linked polymer obtained in the polymerization step is dried; a surface treatment step in which the hydrogel cross-linked polymer or a dried hydrogel cross-linked polymer is subjected to a surface treatment; and a sorting step, carried out after the drying step, in which black or brown particles of foreign matters dotted in a white water absorbent resin powder having a yellow index (YI) value of ≤ 20 are removed from the white water absorbent resin powder entirely or partially based on their colors using a sensor, wherein the sensor determines whether or not there is any coloring by using the color (bright) of a non-defective product sample as a background color, setting a signal at the time when the sensor monitors the background color as a standard and determining a certain standard signal range, comparing the color of a defective product sample and the background color with each other, and rating a signal deviating from the standard signal range as a color difference, and wherein an air jet is emitted and a digital image process is carried out so as to color-sort the foreign matters. <EOS>