Electro-optic (EO) effect is a good candidate to realize an electric-field probe that contains no metal. The EO effect is a phenomenon in which the natural refractive index of a material changes when an electric field is applied. A change in the refractive index that is directly proportional to the electric field strength, which is known as the Pockels effect, theoretically provides a widely flat response in the frequency band from a few megahertz to terahertz. The potential of the optical probe for an over-terahertz measurement was demonstrated with an EO sampling system consisting of a pico-second pulse laser and free-space optics, which characterized a high-speed semiconductor circuit in the time domain [5], and free-space time-domain detection was reported [6]. The application of fiber optics improved the concept of the freely position able probe and eliminated the inconvenience associated with freespace optical propagation [7-10]. We demonstrated the feasibility of the optical probe in a SAR measurement in 2005 [11] and improved the flexibility of the optical fiber-mounted probe for practical use [12-14]. In this paper, Section 2 describes the electric-field measurement system with our developed probe and presents measurement results and the discussion of frequency response