In mountain tunneling, construction works are in the proximity of tunnel faces where falling rocks and collapse of ground are often feared. It is however difficult to improve quickly such work environments e.g., by applying remote construction and complete mechanization. In reality, an exclusive watch guard stands in place and supervises instability of the tunnel face, when charging/connecting explosives and installing tunnel supports. When the guard judges that the continuation of tunneling work is dangerous, he gives warning to the workers, so they can immediately leave from the work area. A newly-developed face monitoring system applies a high-speed camera set beside the watch guard and the image recognition technology. It promptly arrests fall of pebbles and pieces of sprayed concrete, symptoms of possible tunnel face collapse, that could lead to disaster, gives warning within a mere 0.1 seconds after the detection of movements of falling objects, and gets the construction workers to leave immediately from the dangerous spot. Effectiveness of this system in securing safety by foretelling falling rocks was verified in a mountain tunneling site.