Worker was moving MRS unit out of area where mining was completed. He stopped the MRS & walked toward it. When he was appr 8' from front of MRS & about 4' away from ribline a piece of top part of rib broke loose & fell about 6' striking victim on left side of upper torso. He finished working shift. Accident occurred in xcut. Mine ht was about 11 1/2'. He declined med treatment.