We experienced a case of simple pneumonediastinum showing the findings of pleural effusion. Frontal chest radiography showed not only pneumomediastinum but also diffuse haziness with sharp medial margin on left lower lung field, blunting of costophrenic angle, and indistinct contour of diaphragm simulating pleural effusion. CT scan confirmed that these findings arose from the displaced pleura and the associated compression atelectasis by loculated air on the anterior mediastinum.