A 51-year-old woman with a chief complaint of micturition pain and sensation of incomplete voiding was suspected of suffering from a bladder tumor, according to the findings of cystoscopy and ultrasonography. Transurethral punch biopsy of the submucosa of the bladder wall revealed eosinophilic infiltration without malignancy. Conservative treatment with corticosteroids resulted in excellent relief of symptoms and objective remission of the bladder lesions. However, her symptoms recurred 11 weeks after finishing the treatment. She was then treated with a combination of corticosteroid and suplatast tosilate, followed by monotherapy with suplatast tosilate. The treatment was effective for the improvement of symptoms, and serum immunoglobulin E and blood eosinophil levels were reduced. No disease progression was noted after the treatment with suplatast tosilate. To our knowledge, this is the first case of eosinophilic cystitis treated with suplatast tosilate.