  The detailed numerical simulations of the IV-characteristics of Josephson
junction under external electromagnetic radiation show devil's staircases
within different bias current intervals. We have found that the observed steps
form very precisely continued fractions. Increasing of the amplitude of
radiation shifts the devil's staircases to higher Shapiro steps. The algorithm
of appearing and detection of the subharmonics with increasing radiation
amplitude is proposed. We demonstrate that subharmonic steps registered in the
famous experiments by A. H. Dayem and J. J. Wiegand [Phys. Rev 155, 419 (1967)]
and J. Clarke [Phys. Rev. B 4, 2963 (1971)] also form continued fractions.
