Provided is a diluent used for preserving highly-fertile sperm. The diluent is cheaper than that of the prior art and enables the quality of sperm to be improved. In addition, stable quality control of the diluent is possible. Further provided is a method for preserving sperm using the diluent. As a result of refrigerating or freezing sperm for the purpose of preservation using this diluent for sperm, which is characterized by being an aqueous solution containing oligosaccharides, including at least one of fructooligosaccharides, isomaltooligosaccharides, gentiooligosaccharides, and galactooligosaccharides, it is possible to reduce costs while improving the quality of sperm after preservation, and it is possible to provide highly-fertile sperm.
