<p>In order to synthesize a human-friendly flexible machine with a simple structure, multi-directionally flexibly constrained revolute pair (MFCRP), which is a revolute pair with a flexible kinematic constraint in multiple directions, has been proposed as a novel type of a kinematic pair. However, since the relative motion between two links of the MFCRP is limited to the one pattern, the concept of MFCRP is extended as a "flexibly constrained pair" (FCP) and its design method to achieve the specified relative motion including translational motion is proposed. Firstly, a method to specify the relative motion between the links is proposed, where DOF between the links is divided into main-DOF and sub-DOF and the main relative motion is specified. Next, a design method of the flexible constraint to achieve the specified difference in stiffness between the main-DOF and sub-DOF is proposed, here linear springs are optimally arranged to reduce stiffness in main-DOF and the specified non-linear stiffness is implemented in sub-DOF by designing the contact surfaces between the links. Some examples of the FCPs including MFCRP are designed with the proposed design method as demonstration. Then, an example is prototyped and examined by some experiments to confirm validity of the proposed design method.</p>