  We consider the rotation number $\rho(t)$ of a diffeomorphism $f_t=R_t\circ
f$, where $R_t$ is the rotation by $t$ and $f$ is an orientation preserving
$C^\infty$ diffeomorphism of the circle $S^1$. We shall show that if $\rho(t)$
is irrational $$\limsup_{t'\to t}(\rho(t')-\rho(t))/(t'-t)\geq 1.$$
