Human steady running is modeled using a spring-loaded inverted pendulum (SLIP). However, human pushes off the ground actively when starting to run. In this study, we describe a knee joint mechanism for coping with both of an active pushing-off and joint stiffness needed to continue running. To achieve this, knee is equipped with a mechanism comprising a worm gear that improves torque transmission efficiency in order to achieve active movement and two laminated leaf springs for mimicking joint stiffness. We evaluated the performance of the laminated leaf spring and performed an experiment in which the developed running robot started to run. Using the proposed mechanisms, this robot could accomplish hopping with an active pushing-off motion and continued to run using its joint elasticity.