A 57-year-old man visited our hospital with complaint of chest oppression. The plain chest roentgenogram revealed a smoothly margined homogenous mass shadow on the right cardiophrenic angle. Bronchography, computed tomography and ultrasonography revealed intrathoracic cyst. The cyst was completely removed by operation. Resected specimen was multilocular cyst containing serous fluid with 8.5x8x4 cm in size. Pathological diagnosis was benign pleural cyst. With the operation, his chief complaint disappeared. Pleural cyst is a malformation arising from parietal pleura, and only 28 cases were reported in the world. These reports pointed out that it was difficult to diagnose this lesion preoperatively. In our case, ultrasonogram and computed tomogram could detect the cyst. As the pleural cyst is benign, not surgical operation but puncture and follow-up would be the treatment to be recommended. In these points, ultrasonography would be a useful method of diagnosis and follow-up.