A working channel device is mountable on the outer circumference of an endoscope, such as an endoscope of the flexible shaft design. The device has a tubular or hose-like working channel and a distal fixing element that can be fixed to the working channel. The distal fixing element can engage around the endoscope in a clamping manner at the endoscope head of the endoscope. A proximal fixing element can be fixed to the working channel, which can engage around the endoscope, preferably at the endoscope grip of the endoscope. A preferably sleeve-like adapter can be mounted at the proximal end of the working channel of the working channel device. The adapter has or forms the proximal fixing element and additionally forms a connection point/interface for the connection of medical devices.