We report a case of adenocarcinoma detected in the right pleural effusion of a 75-year-old woman. Investigations failed to reveal the site of the primary lesion, and the case was treated as primary unknown cancer. The pleural effusion disappeared after chemotherapy; however, as there was serious bone marrow suppression, the clinical course was observed at an outpatient clinic without chemotherapy. A search for the primary lesion was repeated, but it was not found. One year after first admission, a chest X-ray showed left pleural effusion. Adenocarcinoma was detected in the effusion and a tumor mass obtained from the pleural cavity. Ovarian cancer was diagnosed based on the histological, serological and MRI findings. Thus, this was a rare case of ovarian cancer in which the diagnosis was confirmed by repeated evaluation and in which the initial diagnosis had been primary unknown cancer with malignant pleural effusion only.