  We argue that the origin of "FRI/FRI{-.1em}I dichotomy" -- the division
between Fanaroff-Riley class I (FRI) with subsonic lobes and class I{-.1em}I
(FRI{-.1em}I) radio sources with supersonic lobes is sharp in the radio-optical
luminosity plane (Owen-White diagram) -- can be explained by the deceleration
of advancing radio lobes. The deceleration is caused by the growth of the
effective cross-sectional area of radio lobes. We derive the condition in which
an initially supersonic lobe turns into a subsonic lobe, combining the
ram-pressure equilibrium between the hot spots and the ambient medium with the
relation between "the hot spot radius" and "the linear size of radio sources"
obtained from the radio observations. We find that the dividing line between
the supersonic lobes and subsonic ones is determined by the ratio of the jet
power $L_{\rm j}$ to the number density of the ambient matter at the core
radius of the host galaxy $\bar{n}_{\rm a}$. It is also found that there exists
the maximal ratio of $(L_{\rm j}/\bar{n}_{\rm a})$ and its value resides in
$(L_{\rm j}/\bar{n}_{\rm a})_{\rm max}\approx 10^{44-47} {\rm erg} {\rm s}^{-1}
{\rm cm}^{3}$, taking account of considerable uncertainties. This suggests that
the maximal value $(L_{\rm j}/\bar{n}_{\rm a})_{\rm max}$ separates between
FRIs and FRI{-.1em}Is.
