To Demonstrate the Superiority of IMP (0.3% and 1% OPA-15406 Ointment) Versus the Vehicle in Pediatric Patients with AD

1. Screening period is defined as the interval between the day of obtaining informed consent and the day of the baseline visit (0-30 days). The investigator performs a screening examination after obtaining informed consent from the subject's legal guardian (and, if possible, after obtaining consent from the subject). 2. Assessment period (4 weeks double blind treatment period) is defined as the period between the day of baseline visit and the end of Week 4 visit (or the end of withdrawal visit). Subjects who meet the inclusion criteria and do not meet the exclusion criteria at the baseline visit will be assigned to receive 0.3% or 1% OPA-15406 ointment or comparator (vehicle \[Placebo\]). The allocated IMP will be administered to the treatment area from the day of baseline visit twice-daily for 4 weeks. After the baseline visit, examinations will be performed at Weeks 1, 2, and 4. If a subject discontinues the IMP administration between the day of baseline visit and the day of the Week 4 visit, a withdrawal visit will be performed for that subject. 3. Trial period(4 weeks double blind Treatment) The trial period for an individual subject is the period from the day of obtaining the written informed consent from the subject's legal guardian to the day of the Week 4 visit or withdrawal visit. For subjects who miss the Week 4 visit or withdrawal visit, the termination date, will be the date, as determined by the investigator, when the subject is withdrawn from the trial. The trial period does not include a follow-up period for AE. 4. 24 weeks, Open label, long term treatment period To be eligible for long term treatment, subjects must complete the randomized, double-blind treatment. Subjects must be judged by their investigators to have the potential for clinical benefit by longer-term exposure to OPA-15406, they can continue to receive 0.3% or 1% OPA-15406 open treatment for up to 24 weeks based on the inform consent of the subjects.