The purpose of this research is to develop the assistive caster unit of manual wheelchair for step climbing. This unit is easily replaced with the conventional caster, only a spanner is needed. In addition this caster is simple mechanism, and the power source is only a 006P battery, there is no microcontroller. For realizing this simplicity, The proposed caster unit has two functions to assist the step climbing. One is the assistive plates to enlarge the caster radius imaginarily in the confined front area of the wheelchair, and to enable to climb a higher step. The other is the lock function to fix the caster’s swiveling, which realize easy climbing with oblique approach. The lock-function is activated by the detection of the step, and mechanically fixes the caster’s swiveling. We have developed some prototypes of this caster unit, so far. However, the problem is still remained in the step detection method being the trigger of activation of the lock function. Therefore, this paper proposes the improved activation method by detecting the step at an instant of touching it.