Duchenne Muscular Dystrophy (DMD) is inherited neuromuscular disorders due to mutation in the gene that encodes critical muscle protein called dystrophin. Currently, there is no effective treatment option for the disease. A pharmacological approach by promoting mRNA translation regardless of the presence of premature stop codons by nonsense mutation, called the readthrough strategy, has been developing recently for DMD with nonsense mutation. NPC-14 is a candidate compound for the readthrough strategy, since effective readthrough activities were demonstrated in nonclinical studies. This study is a phase II study designed to assess safety, tolerability, and efficacy of NPC-14 in ambulant DMD patients with nonsense mutation that were confirmed by whole genome analysis. These goals will be accomplished by monitoring adverse events by physical examination, cardiac, pulmonary, auditory, balance, and laboratory tests as safety endpoints, and dystrophin expression in muscle biopsy as primary efficacy endpoint, muscle function (NSAA, timed test, muscle strength (QMT, MMT) , dairy activities by lifecorder), and biomarkers as secondary efficacy endpoints. The study is a randomized, double blind, placebo-controlled study in 21 DMD patients. After screening, eligible patients are allocated dynamically to weekly NPC-14 or a placebo (saline) in a 2:1 ratio and will receive study drugs for 36 weeks.