A surface contamination examining device includes a radiation detector 11 and an arithmetic/display device 13 for displaying radiation intensity in form of a counting rate. The arithmetic/display device has a boundary detecting device 10 that detects the boundary of contamination 14 of an object to be measured by a radioactive material while the radiation detector moves along a surface 18 of the object to be measured. The boundary detecting device 10 includes a counting rate storage unit that sequentially stores counting rates Yi at a predetermined time interval, a slope arithmetic unit that computes slopes aM and aN of approximation lines from previous M (where M is an integer of 3 or larger) counting rates Yi including a final counting rate YM and previous N (where N is an integer smaller than M) counting rates Yi including the final counting rate YM, and a boundary determining unit that determines the boundary of contamination of the object to be measured by the radioactive material on the basis of lines of slope aM and aN.