This paper describes a method to evaluate the quality of position solutions of GNSS (Global Navigation Satellite System) without using other external sensors. Receiving radio waves from NLOS(non-line-of-sight) satellites will cause large positioning errors and reduce quality of positioning results. We propose a method to evaluate the quality of position solutions in Carrier-Phase Differential Measurement for GNSS, where the “integer biases” will be intentionally modified. The present author observed that movement direction of the positioning solutions according to the intentional change of the integer bias of a specified satellite. This observation revealed that the movement will not necessarily coincide with azimuth angle of the satellite. This phenomenon is used for the positioning quality evaluation. We experimented quality evaluation using observation data acquired in several environments and examine the method with several thresholds. As a result, this method can evaluate the quality using median for threshold.