The invention discloses an apparatus and method for detecting a motion of an object in a target space. According to the apparatus, the object is located at a distance from an image-capturing device which is configured to measure the distance and to provide a sensor signal indicative of the distance, the sensor signal being decomposable in a decomposition including odd harmonics if the object is at rest. The apparatus includes a determining module configured to receive the sensor signal and to generate at least one motion signal which depends on at least one even harmonic of the decomposition of the sensor signal; and a detection module configured to detect the motion of the object based on the at least one motion signal and to provide a detection signal indicating the motion of the object.