Good's syndrome is a rare condition of immunodeficiency that is characterized by thymoma and hypogammaglobulinemia. A 74-year-old Japanese woman underwent total thymothymectomy for type AB thymoma (2015 WHO classification). She developed recurrent infectious diseases caused by Escherichia coli (bacteremia), Streptococcus pneumoniae (pneumonia and bacteremia) and Pseudomonas aeruginosa (bacteremia) in the year after thymectomy. The serum levels of immunoglobulin were significantly low (IgG 157mg/dL), which suggested that her infectious diseases were associated with Good's syndrome. Although she began receiving intravenous immunoglobulin every four weeks, she died of pneumonia a week after the second administration of immunoglobulin. When physicians encounter patients with recurrent infection who have a medical history of thymoma, the detection of hypogammaglubulinemia can be a key clue to the diagnosis of Good's syndrome.