In the GPS receiver, GPS solutions are calculated by performing GPS measurement using a Kalman filter. The GPS receiver is provided with a computing system which calculates 2DRMS according to an equation: 2DRMS=2×{square root over ((&sgr;H—Kalman+L )2+L +(HDOP×&sgr;UERE+L )2+L )} wherein, &sgr;H_Kalman represents a horizontal component of an estimate error obtained from a diagonal in an error covariance matrix calculated in a mathematical process of the Kalman filter, HDOP represents a horizontal dilution of precision, and &sgr;UERE is a user equivalent range error.