A 55-year-old man suffered from blurred vision of the left eye (visual acuity was counting fingers/5-10 cm) 8 days after undergoing cataract surgery. After thorough ophthalmologic examinations with gram-negative bacillus justified on smear of vitreous specimen, postoperative endophthalmitis (os) was impressed. Intravitreous injection of 1.0 mg of vancomycin, 0.4 mg of amikacin, and 0.4 mg of dexamethasone was given twice and vitreous fluid culture was also performed. However, the inflammatory condition was not well controlled. Because the results of vitreous fluid culture revealed Flavimonas oryzihabitans infection which is sensitive to piperacillin, we performed the 3rd intravitreous injection of 1.0 mg of piperacillin and 0.4 mg of dexamethasone. The inflammatory condition was under control and the visual acuity of left eye improved. For patients with ocular diseases, postoperative endophthalmitis by Flavimonas oryzihabitans is a rare condition. According to the results of this case, intravitreous injection of piperacillin was effective against the pathogen.