We herein describe a case of Wuchereria bancrofti infection in a previously healthy 37-year-old Nepalese man. The patient presented with a history of milky urine with subsequent acute urinary retention lasting for a few days. The presence of microfilariae was confirmed on both peripheral blood and urine smears obtained at midnight. He was conservatively treated with diethylcarbamazine combined with doxycycline. Filariasis was previously endemic in southern parts of Japan, although it has been eradicated. Clinicians should remember filariasis as a potential etiology of urinary retention, especially in cases that may be associated with imported infectious disease.