This paper describes the first observation of mode locking phenomena in the Large Helical Device. In the experiments, a rotation of the m/n = 1/1 mode slowed down and stopped when the plasma approached the stability boundary of the ideal interchange mode. The minor collapse was caused by the growth of the mode just after mode locking. The mode before the locking has a radial structure with no inversion around the resonance, which implies that the mode has an interchange-type structure at least before the rotation stops. The location of the mode after the locking depends on that of the error or residual field in reduced error field operations.