We present a case of rounded atelectasis (RA) observed continuously following decrease of pleural effusion. The collapsed right lower lobe adjacent to pleural effusion changed its shape from wedge-shaped consolidation with air-bronchogram to oval and flat formation at last. The vessels and bronchi curving into RA were detected by CT and we called it horizontal comettail sign. When a mass is detected following decrease of pleural effusion and vessels and bronchi are curving into the mass, the mass may be diagnosed as RA and no further examination may be necessary.