We propose an operation support system with augmented reality technology. The system can be used to reduce the cognitive load of doctors when doing ultrasonic diagnostics in an operation room. Our system displays ultrasonic images in 3D on the basis of 3D position sensing by a pair of a camera and a marker and three simple interaction methods to compensate for limitations in 3D position sensing. The key function of the system is to project virtual organs created by ultrasonic diagnostics onto the real ones. We worked with a medical doctor to do experiments on the prototype system and found that the system was effective.