A low power monostatic pulse borehole radar system is designed and tested for detecting a subversive intrusion tunnel with about 2 m diameter. According to our calculated budget under the condition that the transmitting signal is over 1 V peak with 10 ns full width half maximum, its switching speed becomes faster than 100 ns for detecting an air-filled tunnel nearby the employed borehole. To satisfy the above requirement, the monostatic pulse radar is fabricated with an active circulator using operational amplifiers and a switch for guaranteeing isolation and cutting off the direct signal. Then, the system performance is evaluated in a well suited tunnel test site. The measured data illustrate that the monostatic pulse radar provides the parabolic pattern of a typical tunnel signature comparable to that of a conventional bistatic pulse radar.