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.