The author have observed mucoid retention cyst in the right maxillary sinus of the patient, 41 years old woman, complained discharging of purulent exudate on the right maxillary molar teeth area, and obtained the following conclusions; 1. The mucoid retention cyst in maxillary sinus casts a faint dome shaped shadow into the radiolucent image of maxillary sinus. 2. The mucoid retention cyst in maxillary sinus may be occured without a history of trauma. 3. Intraoral standard films are also valuable for the interpretation of the lesions in maxillary sinus but only extraoral roentgenograms.