A safety apparatus of a work machine includes: obstacle sensors provided in the upper revolving body; a revolving angle detector; an upper-side coordinate position calculation section that calculates an upper-side coordinate position of the obstacle in the upper-side coordinate system using the upper revolving body as a reference; a lower-side judgment area setting section that sets a lower-side judgment area in the lower-side coordinate system using the lower traveling body as a reference; a coordinate transformation section that transforms one of the upper-side coordinate position and the position of the lower-side judgment area into the other coordinate system to unify the coordinate system; a lower-side approach judgment section that judges whether an obstacle exists within the lower-side judgment area in the unified coordinate system; and a safety operation command section that outputs a safety operation command based on the judgment result.