A 73-year-old man, followed for prostatic hyperplasia, developed submandibular gland cancer. Initially, because of the concurrent presence of elevated serum prostate-specific antigen (PSA) and multiple bone metastases, he was clinically determined as having stage IV prostate cancer in addition to stage II submandibular gland cancer, and radical surgery for his submandibular gland cancer was performed first. However, subsequent detailed examinations of the prostate gland showed no prostate cancer, and a diagnosis of advanced submandibular gland cancer with increased PSA and multiple bone metastases was established. Serum PSA is highly specific for prostate diseases and is widely used as a tumor marker of prostate cancer. However, clinicians should be aware that, in patients with non-prostate cancer, the detection of increased PSA and multiple bone metastases does not necessarily indicate the concurrent presence of prostate cancer.