Background: Japanese cedar (JC) pollinosis is the most common seasonal allergic rhinitis in Japan. Standardized JC pollen extract is available for subcutaneous immunotherapy, but this treatment is limited by potentially serious side effects. The aim of this double-blind, randomized comparative study was to evaluate the efficacy and safety of standardized JC pollen extract in a new oral formulation (CEDARTOLEN®, Torii Pharmaceutical Co., Ltd., Tokyo, Japan) for sublingual immunotherapy (SLIT) for JC pollinosis. Methods: A total of 531 subjects with JC pollinosis were randomized into 2 groups at a ratio of 1:1 to receive daily sublingual administration of standardized JC pollen extract with a maintenance dose of 2,000 Japanese allergy units (JAU) or placebo for 2 consecutive pollen seasons. The efficacy was evaluated using the total nasal symptom and medication score (TNSMS) as the primary end point. Secondary end points included the total ocular symptom and medication score (TOSMS) and scores for individual symptoms and medication. Results: The TNSMS was significantly lower (p < 0.0001) in the SLIT group than in the placebo group in the peak symptom period by 18 and 30% in the first and second seasons, respectively. All secondary end points were also significantly lower in the SLIT group in both seasons. No systemic anaphylaxis occurred. Conclusions: SLIT with daily administration of standardized JC pollen extract was effective for improving nasal and ocular symptoms of JC pollinosis and reducing the use of relief medication. The JC pollen extract was well tolerated with only local adverse events.