Imaging findings of laryngeal cyst are generally specific. We herein report a case of laryngeal cyst in which a diagnosis was difficult. The patient was a 66-year-old man experiencing pharyngeal paresthesia. He was referred to our hospital because laryngeal tumor on the right arytenoid was suspected. Computed tomography and magnetic resonance imaging showed the presence of solid tumor 5cm in diameter in the laryngeal cavity. We performed tracheotomy to prevent airway stenosis and a biopsy via an oral approach. The mass was covered with a smooth mucous membrane, so we made an incision at the membrane and resected the lesion partially. The specimen was not malignant, but we were unable to make any further diagnosis. To confirm the diagnosis and determine the treatment approach, we performed surgery via an external approach. After making an incision at the right thyropharyngeal muscle, we discovered the mass inside the thyroid cartilage. The mass had no adhesion to the surrounding organs and was able to be removed. A histological examination of the resected mass showed degeneration of a cystic lesion with an inflamed capsule.