More safety applications will use the vehicle's own position in the near future. It is very important for those position-based applications to know whether the positioning is reliable or not. The conventional kalman filter is good at estimating the positioning error based on gaussian measurement noise. However, GPS multipath is hard to estimate since it depends on nearby buildings in urban areas. We developed an accurate positioning system and its function to estimate the reliability of the positioning result. We tested the proposed method in an urban area and the estimated reliability matched well with the positioning accuracy..