In this paper, a novel imaging system for ultrasonic wave field distribution is described. In our system, a continuous high frequency low intensity probing wave is applied from perpendicular direction to the observing wave so that the phase of the probing wave is modulated instantly by the product of the nonlinear parameter B/A of object and the pressure of observing wave. Then resulting spatially odulated proving wave is detected and demodulated to derive the distribution of the amplitude of the field of the observing wave along the probing beam. The processes are repeated by shifting the position of the probing beam and 2-D image is obtain. This system can be used to get images of ultrasonic field in opaque media such as organs of human body in real time. Several images of ultrasonic field are obtained. Experimental results show the usefulness of this method.