To stably secure the tightened state of a mold by a form tie even when vibration is allowed to act on the mold.

This locking device 1 of the form tie 100 comprises a head part 10 and a pair of elastic leg parts 11 and 11. A through hole 10a formed along the outline of the nut body 103 forming the form tie 100 and having a size capable of storing a nut body 103 is formed in the head part 10. A clearance capable of storing the center part 102a of a holding body 102 forming the form tie 100 is formed between the inner surfaces 11a of the pair of elastic leg parts 11 and 11. Engaging claws 11b elastically engaged with the edge part 102f of a shaft body 101 facing the shaft one end part 101c side which forms the form tie 100 at the center part 102a of the holding body 102 at the position where the nut body 103 is stored in the through hole 10a are formed in the pair of elastic leg parts 11 and 11.