Device for in-vivo measurement of the concentration of a substance contained in a body fluid

A device for the in-vivo measuring of a constituent of a body fluid, the device including a light emitter which emits light in a wavelength range to which the constituent is sensitive and which is implantable in living tissue, and a light detector which, when the device is in a measuring position, receives the light emitted by the emitter and outputs a signal depending on the amount of light received, from which signal the constituent may be measured, wherein the light detector is situated outside the tissue when the device is in the measuring position.