This paper describes a new mechanism that will realize a stair-climbing robot within small size. The mechanism was named "SMART" after the concept of this mechanism, "Simple Mechanism Adaptive for Rough Terrain". Some statical conditions to realize a low height mechanism that is driven by only one motor were derived. The prototype robot that equips a SMART mechanism had been designed and manufactured actually. Experimental results show that the estimated output torque is close to the measured torque. This paper describes the result of analysis of the mechanical efficiency.