A random number generating device includes a particle detector, a pulse generator, a clock counter, and a random number converter. The particle detector detects particles emitted from a radioactive isotope. The pulse generator generates pulses corresponding to the particles. The clock counter counts the number of clock cycles during time intervals between the pulses and generates a plurality of count values. The random number converter adjusts a clock frequency, based on a minimum value and a maximum value of the plurality of count values and converts a target count value generated depending on the adjusted clock frequency into a random number.