In a summer science extension course for children at Shimane University,“Let's learn how to wash the hands.” has held to experience on the procedure of the prevention of infection to elementary and junior high school children. For visual demonstration of washing remnants fluorescence staining Glitter-Bug[TM] has applied, and for numerical demonstration of contamination measurement the ATP-measurement[TM] has applied before and after the lesson of the prevention of infection and hand-washing procedure. Significant reduction of ATP was observed after the lesson. However, washing remnants of 8 children (33.4％) were beyond a criteria-of-control value after the lesson. The attention should be paid for person-focused explanation to each candidate. It was suggested that empowerment of understanding of the children by the experience with combining of visualization and numerical evaluation in Let's learn how to wash the hands. course.