Abstract: A body sensor for measuring body activity of a user includes a sensor part and a separate connecting part for securing the sensor part to another side of a user's body part. The sensor part includes a housing enclosing components of the sensor part inside the housing. The components include a circuit board, an optical sensor configured to measure body activity by optical measurement, a first magnetic coupling part provided in connection with the optical sensor, and one or more other components provided inside the housing. The separate connecting part is independent from the sensor part and includes a second magnetic coupling part configured to provide a magnetic coupling with the first magnetic coupling part via the first surface of the housing.