This paper proposes a controllability measure for
nonlinear systems which evaluates a degree of controllability.
There are many researches on controllability measure for
linear systems. However there exists no measure for nonlinear
systems. This paper introduces a theorem about a property
of systems which are not strongly accessible and proposes
controllability measure based on the theorem. Furthermore
this paper demonstrates that the measure relates to a distance
to a system which is not strongly accessible which is one of
measures of evaluating a degree of controllability. Finally this
paper applies the measure to an example system and calculates
the controllability measure and the distance of the system.