We have studied the simplified DGPS using two GPS receivers, and obtained from 1 to 10m accuracy of point positioning. However, in case when high buildings are nearby, it occurs that sufficient positioning accuracy can't be obtained because of the multipath phenomena. In this paper, the multipath effects in the GPS positioning, and an effective method to reduce the positioning deterioration due to the multipath are described. We also examine the radiation pattern of GPS antenna with metal board which is installed for cutting out the multipath.