A case of a biphasic synovial sarcoma, arising on the inner surface of the anterior abdominal wall of a 13-year-old girl, is reported. Although the tumor showed rather typical histological and immunohistochemical features for synovial sarcoma, its unusual clinical presentation and anatomical location caused diagnostic difficulty, especially with regard to differentiation from a malignant mesothelioma. Applying reverse transcriptase polymerase chain reaction (RT-PCR) analysis, the SYT-SSX2 chimeric gene transcripts that result from the translocation, t(X;18)(p11.2;q11.2), found in most synovial sarcomas could be demonstrated. Thus, this RT-PCR approach is a reliable method for confirming the diagnosis of synovial sarcomas in unusual locations.