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 position control of the ColoBot for guiding the advancement in a tubular, compliant and slippery environment. The kinematic model related the position of the distal end of the ColoBot to the actuator inputs which is firstly developed and formulated to control the shape of the ColoBot through position control of the distal tip. To achieve safe guidance, the ideal position of the tip should be in the central axis of the colon. A method based on a circumscribed circle is proposed to approximate the central position in real-time based on three sensor readings. This position will be used as reference position for the tip to adjust its shape in real time to avoid the contact with tube wall. This proposed approach can be extended to the control of continuum robots in the conditions of a dynamically confined space. The simulation results and experimental results with a curved tube will be presented in order to validate the proposed control strategy.