A 20-year-old man diagnosed as idiopathic scoliosis with Cobb angle 146 degrees was scheduled for two-stage operations. Anterior dissection of the thoracic vertebra in the left lateral decubitus position, and the placement of pedicle screws in the prone position were performed as the first-stage operation. During surgery, the patient developed liver contusion with ascites, probably due to hepatic compression placed between vertebrae and operating table in the prone position. In the second operation for posterior spinal fusion, the occurrence of liver contusion was prevented by performing abdominal ultrasonography before and after surgery, and monitoring AST/ALT during anesthesia as the indicators of liver contusion. Intraoperative management for organ protection is required during anesthesia in patients with idiopathic scoliosis associated with thoracic deformity.