Preanesthetic medication is important to eliminate surgical anxiety in pediatric patients and facilitate their smooth transfer to the operating room. Midazolam is the most commonly used preanesthetic medication. However, it has been reported that the sedative effect varies from patient to patient. In this study, the pharmacokinetics of midazolam were examined, and the aim was to assess the factors affecting the quality of sedation. The participants were children ranging in age from 6 months to 8 years scheduled for surgery. Midazolam 0.5 mg/kg was administered orally 30 min before entering the operating room, and the sedation level was evaluated at the time of mask application. Blood was collected after slow induction, and the serum concentration of midazolam was measured using high-performance liquid chromatography. A total of 98 patients were registered. There was no difference in serum concentrations between the effective sedation group and the ineffective sedation group (48.0 vs. 49.1 ng/mL), regardless of the effect of midazolam. Percentages of ineffective sedation by age (0 to 7 years) were 66.6%, 60%, 33.3%, 11.1%, 0%, 0%, 12.5%, and 0%, respectively. On multivariate logistic regression analysis, siblings (OR = 3.9, CI: 1.1-14.0, p = .03) and age (OR = 3.2, CI:1.2-8.5, p = .02) were related to an insufficient sedative effect. The serum concentration of oral midazolam reached effective levels even in patients in whom the sedative effect was inadequate. It is important to manage the perioperative period with appropriate concurrent premedication taking into account patient age and social background characteristics. Clinical trial registry: UMIN R000052504.