Current allergen-specific immunotherapy (AIT) for pollinosis requires long-term treatment with potentially severe side effects. Therefore, development of an AIT that is safe and more convenient with a shorter regimen is needed. This prospective, double-blind, placebo-controlled trial randomized 55 participants with Japanese cedar pollinosis (JCP) to active or placebo groups to test the safety and efficacy of short-term oral immunotherapy (OIT) with Cry j 1-galactomannan conjugate for JCP. Mean symptom-medication score as the primary outcome in the active group improved 27.8% relative to the placebo group during the entire pollen season. As the secondary outcomes, mean medication score in active group improved significantly, by 56.2%, compared with placebo during the entire pollen season. Mean total symptom score was similar between active and placebo groups during the entire pollen season. There were no severe treatment-emergent adverse events in the active and placebo groups. Therefore short-term OIT with Cry j 1-galactomannan conjugate is safe, and effective for reducing the amount of medication use for JCP.