A reagent for partially lysing a cell membrane of a red blood cell infected with a malaria parasite, comprising: a first surfactant which has predetermined lysing ability for the red blood cell membrane; and a second surfactant which has weaker lysing ability than that of the first surfactant; wherein the reagent has pH of 5 to 7 and osmotic pressure of 200 to 300 mOsm/kg·H2O; and wherein the reagent partially lyses the malaria infected red blood cell membrane such that a malaria parasite is retained in the red blood cell and fluorescent dye passes through the cell membrane, is disclosed. A reagent for detecting malaria infected red blood cells, and a sample analyzing method for detecting malaria infected red blood cells, are also disclosed.