The invention relates to an apparatus for measuring blood oxygen saturation, comprising a housing (1), a sensor (2), a connecting cable (3) and a plug (4), wherein the housing (1) has a cavity (5) for accepting a patient's tissue which is supplied with blood; the sensor (2) which is arranged on the housing and has at least one light source (21) for the emission of light which passes through the tissue which is supplied with blood and a detector (22) for receiving the light passing through the tissue which is supplied with blood; and the apparatus can be connected to an evaluation unit via the plug (4).