Due to their compliance and high dexterity, biologically-inspired continuum robots have attracted much interest for applications such as medical surgery, urban search and rescue, de-mining etc. In this paper, we will present an application to medical surgery-colonoscopy by designing a pneumatic-driven flexible robotic manipulator, called ColoBot. The focus of this paper lies in the sensor-based guidance control of the ColoBot in a tubular, compliant and slippery environment of human colon. The kinematic model related the position and orientation of distal end of ColoBot to the actuator inputs is firstly developed and formulated for orientation control of ColoBot. For the autonomous guidance inside the colon, a method based on a circumscribed circle is utilized to calculate the real-time reference paths from the measurement of three sensors for orientation control of the Colobot. This proposed approach can be extended to the control of continuum robots in the conditions of a dynamically confined space. The experimental results on an emulation platform will be presented in order to validate the proposed control strategy.