There are two types of devices to assist physical motions: passive and active. Passive devices can be inexpensive and lightweight, however, require a mechanism for controlling the output. We suggested a mechanism in which tensile and compression springs were placed on the front and back of the thigh, in order to assist knee extension during squatting and to avoid disturbing during walking. We performed a simulation to confirm whether the desired output control could be achieved, and a prototype was fabricated. The simulation results indicated that the output characteristics could be changed by the dimensional design, and that the desired control could be achieved. However, the prototype did not achieve the expected results because it was not compact and lightweight, which is an advantage of passive devices. Our future work is to improve the detailed design to reduce the size and weight.