A contact probe includes a stylus and an optical detector configured to detect a posture of the stylus optically. An illumination subject portion is formed on the stylus and has three or more reflection surfaces. The optical detector includes three or more fibers, a light source, a condenser lens group, and a wavelength detector. The wavelength detector calculates posture information of the stylus on the basis of wavelength variations of reflection light beams that are caused by variations of intervals between the condenser lens group and the three or more reflection surfaces, respectively. The contact probe acquires coordinates of a position of the contact to the object to be measured on the basis of posture information obtained by the optical detector.