A vehicle direction estimation section estimates an absolute direction of a heading direction of a vehicle based on GPS information and vehicle information. A satellite direction estimation section estimates an absolute direction of a target satellite of several satellites for positioning based on corresponding GPS information, and estimates a relative direction of the target satellite with respect to the heading direction based on the estimated absolute direction of the heading direction and the estimated absolute direction of the target satellite. A shield determination section determines a shielded state of the target satellite by determining whether a signal strength of a satellite signal from the target satellite is greater than a threshold value relative to a target partition of several partitions into which a three-dimensional sphere with respect to the heading direction is divided, the target partition which the calculated relative direction of the target satellite belongs to.