The present invention relates to a diluent for a Norovirus or Sapovirus specimen, the diluent containing an alkaline buffer of pH 9.0 to 10.0, and to a method for detecting a Norovirus or a Sapovirus through use of the diluent. According to the present invention, a Norovirus or a Sapovirus can be detected in a Norovirus- or Sapovirus-containing specimen such as stool, vomit, body fluid, blood, body tissue, or food, in a convenient manner without use of a special machine such as a centrifuge, with an improved detection sensitivity, and with complete elimination of non-specific factors.