An apparatus for monitoring a spatial area, in particular for safeguarding a hazardous area of an automatically operated installation, comprises an illumination device which at least temporarily emits light signals into the spatial area. A first image recording unit records a first image of the spatial area. The first image recording unit comprises an image sensor having a plurality of pixels. An evaluation unit determines a distance value for at least one spatial area point, which is located in the spatial area and is imaged on at least one pixel, by means of a propagation type measurement. The propagation type measurement suffers from a limited unambiguity range. Therefore, a test device is designed to check the distance value by means of a reference distance value determined from a second image of said spatial area.