A random number generating circuit comprises: the seed generating circuit which generates a seed; and a pseudo random number circuit which generates pseudo random numbers based on the seed generated by the seed generating circuit. The seed generating circuit has: an oscillating circuit which oscillates continuously or intermittently, and which outputs a digital data sequence; a smoothing circuit which outputs time series data by controlling appearance frequencies of “0” and “1” in the digital data sequence outputted from the oscillating circuit; and a postprocessing circuit which generates one-bit seed by a computation using a plurality of bits included in the time series data.