Diffuse esophageal spasm is a rare condition, which has generally been treated conservatively in Japan. A case of this disorder treated successfully by long myotomy is reported with a knowledge of prolonged pressure monitoring of the esophagus. A 56-year-old woman was admitted to our hospital with dysphagia. Barium swallow and esophagoscopy showed contraction of the esophagus, and manometry showed normal peristalsis, so the diagnosis of achalasia was ruled out. Prolonged pressure monitoring of the esophagus showed spastic contractions with the pressure over 130 mmHg that continued for 25 sec during meals, which led us to the diagnosis of diffuse esophageal spasm. Extramucosal long myotomy of the esophagus and a modified Belsey Mark IV operation were performed. The postoperative course was satisfactory and esophageal functional tests showed no spasms. The patient regained weight without dysphagia.