The present paper and the companion paper describe kinematic syntheses of planar link mechanisms with only revolute pairs to develop leg-mechanisms of biped machines which walk straight on flat grounds. It has been proposed to divide the leg-mechanism into two sub-mechanisms in order to make a kinematic synthesis possible ; one is an ankle-path-generator which generates a required trajectory of the ankle relative to the upper body, and the other is a foot-driving mechanism which generates a require angular motion of the foot about the ankle joint. The 1st paper discusses type selections of the ankle-path-generator, conditions and procedures of dimensional syntheses and the characteristics of the synthesized mechanisms, resulting in three practical ankle-path-generators.