Pair of engagement chains (100) whose buckling strength is improved, which realizes stable forward and backward driving for a work without requiring to install an auxiliary guide means for suppressing buckling and inclination of the engagement chains (100) and whose backlash prone is eliminated. The pair of engagement chains (100) has inner tooth plates (110), bushes (120), outer tooth plates (140) and connecting pins (150) and is integrated by engaging hook portions of the inner tooth plates (110) and hook portions of the outer tooth plates (140), respectively, and is branched by disengaging the hook portions of the inner tooth plates (110) and the hook portions of the outer tooth plates (140), respectively. The pair of engagement chains (100) further includes inflection restricted plates (170) having inflection restricted surfaces (171) and inflection restricting plates (180) having inflection restricting portions (181) that are connected on the both ends of the connecting pins (150).