It is difficult for sellers of used cars to expect the contract prices at auto auction because contract prices depend on individual conditions such as mileage, model year, body color, etc. even if used cars are the same model. Similarly, it is necessary for buyers to expect the contract prices when buying used cars from customers. In this study, we used the regression tree and the regression equation to analyze the influence of several factors on contract prices. As a result, in addition to the main factors such as mileage and model year, it was found that vehicle color and regionality also affect contract prices at auto auction.