PURPOSE: To prevent the breakage of the couplers of concrete structure by using a flexible coupler in which a piece stop whose end is coupled with the joint bars such as threaded structural bar and an elastic block covering the screw are housed in a joint casing.

CONSTITUTION: Joint casings 4 are buried in the recessions 10a of joint walls 10 of box culvert in compliance with concreting work, and grooves for joint bars 2 are provided in the middle portion of the casing 4. A stop piece 1 in a state of being buried in an elastic block 3 is set in the casings 4, and both threaded ends of the joint bar 2 such as steel bar in a condition that a buffer rubber 6 is put between concrete walls 10 are coupled with the pieces 1. The peripheral side of the block 3 is pressingly contacted with the joint casings 4 and clamped, and finally the recessions 10a are filled with mortar after attaching flexible couplers.