Herein, we have developed a humanoid robot that achieves dynamic motion. Focusing on the running motion that is the basis of the motion, the robot has been developed focusing on the pelvic rotation on the frontal plane and the elasticity in leg joints (that changes according to running speed), which are the characteristics of humans during running. However, the variable joint stiffness mechanism that we have developed was large and heavy. Therefore, to make the mechanism smaller and lighter, we shorten the length of the leaf spring. We succeeded in downsizing the mechanism by changing its rectangular shape to trapezoidal, while maintaining strength and elasticity. The variable joint stiffness mechanism thus developed was more flexible, and its weight was reduced from 1.9 kg to 0.7 kg. The mechanism was mounted on the ankle joint, and it was confirmed that the required specifications were satisfied.