A 27-year-old female patient underwent cesarean section and a postoperative hematoma occurred at the site of the uterine incision. The patient underwent relaparotomy to remove the hematoma. Four days later she developed a fever of over 39°C and an abscess had formed at the site. Despite therapy with several antimicrobial agents, her fever persisted. Consequently, she underwent transvaginal abscess drainage, after which she promptly became afebrile. Mycoplasma hominis was considered to be the primary causative organism. There are two reasons that could explain why the wound infection became serious: (i) M. hominis is resistant to several antimicrobial agents that are usually used to treat obstetric infections; and (ii) a long time is required to identify the pathogen. In conclusion, M. hominis should be considered as a causative organism if an antimicrobial-resistant infection occurs at the surgical site after a cesarean section.