We report two cases of accidental overdoses of intramuscular midazolam used for a conscious sedation. A 4-year-old boy with dental caries was scheduled for treatment under conscious sedation. The pedodontist prescribed midazolam (dormicum® 5 mg / 5 ml) 2 ml (2 mg) by verbal order to hygienist. The hygienist instead of the pedodontist wrote a prescription for midazolam (dormicum® 15 mg / 3 ml) 2 ml (10 mg). The inexperienced nurse gave an injection to his buttock as prescription. The child fell into a deep sedation. A 4-year-old boy with dental caries was scheduled for treatment under conscious sedation. The inexperienced pedodontist gave an injection to his buttock midazolam (dormicum ® 15 mg / 3 ml) 3 ml (15 mg) instead of midazolam (dormicum® 5 mg / 5 ml) 3 ml (3 mg). The child fell into a deep sedation. Both cases had no complications, but the accidents happened as a result of the inexperienced dental staffs. The five times midazoalm instead of the intended doses was inadvertently given intramuscularly, fortunately caused no harm in our cases. However, the situations suggest that we should carefully check the dosage and review the correct procedures, even when using a drug that is considered to be familiar with most practitioners.