A 96-year-old female visited our hospital with a complaint of right flank pain. She was under treatment for diabetes mellitus, hypertension and moderate renal dysfunction in the department of internal medicine. Computer tomographic (CT) scan showed a mass 12 cm in diameter compressing the right kidney and inferior vena cava. We diagnosed the right renal cyst. Eight weeks later, CT scan revealed a gradual increase of the mass. We punctured the tumor in order to reduce her complaint and investigate the rumor. Because contrast material could not be infused into the mass, needle biopsy was performed. Pathological examination revealed hypercellularity of spindle cells, that showed positive for CD34, vimentin, desmin and Mic-2, and negative for S100, alphaSMA, c-kit, AE1/3, p53 and bcl-2. The tumor was finally diagnosed as solitary fibrous tumor.