Abstract: An apparatus for minimally invasive length measurement within a hollow organ is provided. The apparatus has an endoscopic instrument, an ultrasound head having an ultrasound transmitter and ultrasound receiver, an ultrasound reflector, an electrical connection, a control and display device and a mechanical connection. The ultrasound reflector is arranged at one end of the endoscopic instrument. The ultrasound head is arranged at one end of the mechanical connection extending into a channel of the endoscopic instrument and is guided herein. The ultrasound head is aligned with the ultrasound reflector and the distance between the two can be changed. The ultrasound head and the control and display device are connected by the electrical connection to exchange electrical signals. The control and display device generates an ultrasound wave and detects the wave reflected at the ultrasound reflector using the ultrasound receiver and determines the delay time of the wave.