Weight aids system for Horse Riding Robot Simulator A rider communicates with a horse to control the horse. This type of communication, collectively known as riding aids, is important in horse riding. When the rider communicates with the horse during horse riding, the horse determines the rider’s command with its own intelligence and controls its own motors. We expect that probability factors can represent the riding aids and they can be used to control the horse. Therefore, we propose a probability model of riding aids, especially weight aids. To estimate the probability of the weight aids, we devised a sensor-equipped saddle that measures the vertical force. These sensors calculate the Center of Gravity (COG). In experiments, the rider rides on the horse with the sensor-equipped saddle and the sensors record the COG data depending on the rider’s commands. Finally, we formulated the probability of the weight aids as rider commands. The probability of weight aid can be a prior probability of other riding aid probability, and we expect that the rider can control a horse robot simulator with theses probability models.