The purpose of this study is to verify the efficacy of NPC-15 (melatonin 1 mg/d or 4 mg/d) versus placebo to sleep latency with electronic sleep diaries.

This study will be a multicenter, double-blind, randomized, placebo-controlled parallel group trial. The trial has five phases; the pre-screening phase, the screening phase, double-blinded treatment phase, open label treatment phase and post-treatment phase. The screening phase comprises a screening visit where subject's initial eligibility will be evaluated. During double-blinded treatment phase, patients who meet all entry criteria for the trial will be randomized into three groups, and they will be assigned NPC-15 1 mg, 4 mg or placebo before going to bed for 2 weeks. During open label treatment phase, all patients will be administered NPC-15 1 mg, 2 mg or 4 mg on the basis of their doctors' judgements.

Inclusion Criteria:

• Female or male patients aged 6 to 15 years.
• Patients with "autism spectrum disorder" diagnosed by using DSM-5.
• Patients with average of daily sleep latency persisted over 30 min and the condition is continuous over 3 months
• Patients who are out-patient, not hospitalized patient.
• Signed informed consent obtained from rearer(s)/parent(s)/guardian(s) of the patient, or signed IC or informed assent obtained from the patient themselves.

Exclusion Criteria:

• Patients with at least severity in either Conceptual area, or Social area, or Practical area of intellectual disability have judged more than "severe" by using DSM-5.
• Patients who took melatonin (including supplement) in history.
• Patients who had taken Ramelteon within 4 weeks before clinical study starts.