An apparatus (10) for preoperative planning of an operation of replacing artificial knee joint, comprising: input means an image which introduce a two-dimensional tomographic image of the lower limb including the knee joint； means (S101) for reconstructing an image which reconstructs the three-dimensional image of the femur and tibia from the image input by the input means of an image； means (S103) to determine the artificial joint femur side artificial joint which determine to be replaced from the three-dimensional image of the knee joint of the femur obtained by the means of reconstruction of an image； means (S106) to determine the artificial joint side of the tibia which determine the artificial joint to be replaced from the three-dimensional image of the knee joint of the tibia obtained by the means of reconstruction of an image； and means (S113-S115) for determining parameters which determine various parameters used in the replacement of artificial knee joint that use an alignment rod in the marrow which is inserted into the femur (S112) based on the artificial joint determined by the determining means of the artificial joint side of the femur and a reference point of the joint characterized knee because the various parameters to be determined by the means for determining parameters include the insertion position, the insertion orientation and the insertion depth of the alignment rod in the bone and the position and rotation angle of the surface of the bone cutting joint part based on the inserted alignment rod in the marrow.Un aparato (10) para la planificación preoperatoria de una operación de sustitución de articulación artificial de rodilla, que comprende: medios de introducción de una imagen que introducen una imagen tomográfica bidimensional de la extremidad inferior incluyendo la articulación de rodilla； medios (S101) para reconstruir una imagen que reconstruye la imagen tridimensional del fémur y la tibia desde la introducción de la imagen gracias a los
