As a position sensing probe for Nano-CMM which measures three-dimensional shapes of microparts, we propose a novel probing technique using circular motion of an optically trapped microsphere. In this report, a fundamental principle is described for sensing a coordinate on a work surface using a circular motion probe. The circular motion of the trapped sphere near a work surface becomes an ellipse compressed perpendicularly to the surface due to the change of viscous drag of the sphere. The elliptical orbit of the trapped sphere depends on a distance from the surface and a normal vector direction of the surface. By processing the elliptical orbit, the circular motion probe can detect a position and a plane normal vector of the work surface simultaneously. In order to verify feasibility of this method, fundamental experiments are carried out. The circular motion probe is approached to a vertical silicon cleavage surface. The behavior of the trapped sphere near the surface agrees well with the theory. Based on the elliptical orbit of the trapped sphere near the surface, a position and a plane normal vector of the surface are estimated. It is verified that the circular motion probe can detect a position of a work surface with resolution of better than 50nm and detect a plane normal vector of the surface.