The invention relates to a medical apparatus (120,130) having a sensor (100,110) for detecting a force acting on the medical apparatus (120,130) in a longitudinal direction. The medical apparatus (120,130) comprises an opto-mechanical force transducer having a flexible part (22,42) for receiving the force, an optical guide (1) having an outcoupling surface (11) that faces the flexible part (22,42) of the opto-mechanical force transducer, and a photodetector which detects an interference pattern composed of light (32) in the optical guide (1) that is reflected from the outcoupling surface (11) of the optical guide (1) and of light (33) in the optical guide (1) that is reflected from the flexible part (11) of the opto-mechanical force transducer. The use of the interference pattern composed of the light reflected from the out-coupling surface of the optical guide and the light reflected from the flexible part of the opto-mechanical force transducer results in a more accurate measurement of the force acting on the medical apparatus.