Primary cardiac liposarcoma is very rare and usually asymptomatic. It is often diagnosed at an advanced or incurable stage without being presented in surgical operation. We report a man in his early sixties with primary cardiac liposarcoma originating from the ventricular septum. The malignant nature of the tumor was suspected by positron emission tomography using fluorine-18 fluorodeoxyglucose as the tracer. The final diagnosis was made histopathologically following surgery. Treatment with carbon ion radiotherapy was applied, but failed to induce tumor regression.