Lectin for checking a blood type is extracted from seeds of bitter gourd, balsam pear, or Momordica charantia. A blood type checking reagent includes the lectin. The lectin extracted from seeds of bitter gourd preferably has a molecular weight between 100,000 and 170,000 measured with poly-acrylamide gel electrophoresis in existence of sodium dodecyl sulfate (SDS-PAGE). A method of checking a blood type includes the steps of: extracting lectin from seeds of bitter gourd, balsam pear, or Momordica charantia; preparing a blood type checking reagent containing the lectin; and checking a blood type using the blood type checking reagent.