Flexible robots have been widely studied for application to minimally invasive surgery because of their dexterity and miniaturization ability. Conventional master devices developed to manipulate rigid link-type robots are difficult to apply to flexible robots due to structural difference. In addition, the different kinematic structures of the master device and flexible slave robot cause complex mapping issues. Furthermore, most high-redundancy flexible robots inserted through an over-tube have limited bending angles depending on the insertion depth. Conventional master devices were not designed to comply with this limitation of flexible robots. We developed an isosceles master device that provides intuitive and simple mapping factors for controlling and maintaining the wire tension of a flexible robot in a safe range. We applied a variable isosceles triangle mechanism that structurally limits the bending angle according to the insertion depth. Experimental results showed that our master device can control a flexible slave robot with an error of less than 1.5 mm. Because of the limited bending angle, excessive tension and the resulting damage to the wire were prevented. The isosceles master device can also hold its position and orientation with a passive holding mechanism.