An engagement chain type advance/retract actuator comprising: a pair of engagement chains in each of which an inner link unit composed of hook-shaped inner tooth plates arranged spaced apart in a left and right pair is plurally coupled in a chain longitudinal direction by means of pairs of front and rear coupling pins press-fitted to pairs of front and rear pin holes of hook-shaped outer tooth plates adjacently arranged outside the inner link unit , the pair of engagement chains being integrated by meshing the inner tooth plates together facing each other while tilting against each other and the outer tooth plates together facing each other while tilting against each other, respectively, the pair of engagement chains being furcated by disengaging the mutually meshed inner tooth plates and the mutually meshed outer tooth plates , respectively; a drive sprocket advancing and retracting the pair of engagement chains ; and a driven body mounted on chain distal ends of the pair of engagement chains via a mounting member (C) and driven according to advancing and retracting movement of the pair of engagement chains , the actuator characterized in that : the pair of engagement chains have a chain stiff part in which the inner tooth plates and the outer tooth plates are mutually meshed constantly in a predetermined range from the chain distal ends along a stiffening direction of the pair of engagement chains ; and a reinforcing core reinforcing the chain stiff part is inserted and fitted in a prismatic hollow part (S) of the chain stiff part . a pair of engagement chains in each of which an inner link unit composed of hook-shaped inner tooth plates arranged spaced apart in a left and right pair is plurally coupled in a chain longitudinal direction by means of pairs of front and rear coupling pins press-fitted to pairs of front and rear pin holes of hook-shaped outer tooth plates adjacently arranged outside the inner link unit , the pair of engagement chains being integrated by meshing the inner tooth plates together facing each other while tilting against each other and the outer tooth plates together facing each other while tilting against each other, respectively, the pair of engagement chains being furcated by disengaging the mutually meshed inner tooth plates and the mutually meshed outer tooth plates , respectively; a drive sprocket advancing and retracting the pair of engagement chains ; and a driven body mounted on chain distal ends of the pair of engagement chains via a mounting member (C) and driven according to advancing and retracting movement of the pair of engagement chains , the actuator characterized in that : the pair of engagement chains have a chain stiff part in which the inner tooth plates and the outer tooth plates are mutually meshed constantly in a predetermined range from the chain distal ends along a stiffening direction of the pair of engagement chains ; and a reinforcing core reinforcing the chain stiff part is inserted and fitted in a prismatic hollow part (S) of the chain stiff part . the pair of engagement chains have a chain stiff part in which the inner tooth plates and the outer tooth plates are mutually meshed constantly in a predetermined range from the chain distal ends along a stiffening direction of the pair of engagement chains ; and a reinforcing core reinforcing the chain stiff part is inserted and fitted in a prismatic hollow part (S) of the chain stiff part . <EOS>