An array type directional borehole radar system employing passive sensors, optical electric field sensors was developed. The system uses a dipole antenna for transmission and four optical electric field sensors for reception. Four sensors are evenly arranged in the circumferential direction to constitute an antenna array. Although an optical electric field sensor itself does not have directivity, azimuth angle of a target can be known from phase differences among the four sensors. Difficulty of this approach lies in the fact that, due to the size of a borehole, the spacing between sensors is limited to a few centimeters that is significantly smaller than the wavelength. As a result, phase difference to be extracted is small and easily subject to noise. In addition, mutual coupling between the antenna elements may causes distortion of target signal. In this paper, we discuss feasibility of target azimuth detection. A laboratory experiment was carried out to examine capability of extracting a small phase difference and to evaluate the mutual coupling between antenna elements.