Oral α-hemolytic Streptococcus contains etiologic agents that can cause not only dental caries, but also bacterial pneumonia, infective endocarditis and other diseases. As bacteria on the tooth surface can be physically removed through brushing, it is necessary to train children to acquire the proper toothbrushing method in the infancy stage of forming a habit of brushing their teeth. This study was conducted with the objective of clarifying the changes in the number of a hemolytic Streptococcus adhering to the tooth surface before and after self-brushing by infants, with the intention of being useful in terms of the oral hygiene environment, developing measures to prevent pneumococcal infection and establishing toothbrushing habits. The participants, after obtaining parental agreement, consisted of 117 nursery school-age children (ages 3, 4 and 5 years) with an average age of 60.5±9.9 months. The bacteria count, irrespective of the infants' sex, was significantly lower after brushing than before brushing. However, as the count was 6.1 x 10⁴ CFU/mL before brushing and 3.1×10⁴ CFU/mL after brushing, the need for instruction to reinforce infants' self-care agency is indicated. The bacteria count before brushing among 3-year olds tended to be higher than among 4-year olds, suggesting that the 3-year olds' ability at fine movement is not yet developed. No significant correlation was determined between brushing time and reduction in bacteria count, and it is considered that if proper brushing is performed even over a brief period, the bacteria count can be reduced. Moreover, the bacteria count before brushing among infants without the presence of fathers and infants with a sibling was significantly higher, pointing out the need to recognize them as a high risk group from the viewpoints of dental caries and pneumococcal infection prevention.