This paper proposes a controllability measure for nonlinear systems which evaluates a degree of controlability. First, this paper focuses on an property of systems which are not strongly accessible and introduces a theorem about an annihilator of a strong accessibility distribution of nonlinear systems. Next, a controllability measure based on the theorem is proposed. 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 controllabiliy. Finally this paper applies the measure to an example system and calculates the controllability measures and the distance of the system.