Disclosed is a method of generating secret information on the basis of a ring oscillator. According to an embodiment of the present disclosure, there is provided an apparatus for generating secret information on the basis of a ring oscillator, the apparatus including: multiple PUF information generation units each including at least one ring oscillator cell and generating physically unclonable function (PUF) information generated by the at least one ring oscillator cell; a phase checking unit cross-checking phases for the multiple pieces of the PUF information that are output from the multiple PUF information generation units, respectively; and a secret key generation unit outputting secret key information based on a result of comparing the multiple phases received from the phase checking unit.