PROBLEM TO BE SOLVED: To provide a method capable of enhancing the accuracy of measurement by considering factors which affect the measurement of a space dose rate in the vicinity of a nuclear power facility after an accident and thus capable of measuring a deposition quantity of radioactive cesium on a ground surface more accurately.SOLUTION: A space dose rate conversion coefficient for converting a gamma ray counting rate detected by a radiation detector mounted on an unmanned helicopter to the value of a space dose rate is calculated in advance; an attenuation coefficient of the gamma ray counting rate by air is calculated in advance from the relationship between a plurality of ground altitudes at one previously specified point and the gamma ray counting rate; a gamma ray counting rate at the certain point of time when the unmanned helicopter flies at a certain ground altitude in the vicinity of a nuclear power facility and positional information at this point of time are obtained by the radiation detector
