Computer-assisted surgery (CAS) is used in various clinical settings. The introduction of CAS has improved accuracy and safety by reducing the duration of surgery. An 86-year-old man was referred to our hospital for treatment of a mandibular fracture. Considering his advanced age, it was desirable that the surgery duration should be as short as possible. Virtual surgical planning was performed with computer software based on the preoperative computed tomography (CT) data. Mandibular segments were reconstructed as an ideal configuration of the patient's mandible. Finally, a 3-dimensional (3-D) life-size model of the patient's mandible was constructed with a 3-D printer based on the simulation data using a rapid prototyping method. A titanium reconstruction plate was bent to adapt to the simulated mandibular model preoperatively. During surgery, the prepared plate was fitted to the mandible with minor adaptations. The surgery was completed uneventfully. CAS was effective in forming the ideal configuration of the mandible with reduced operating time and less blood loss. Therefore, this method is useful for the treatment of fractures of the atrophic mandible.