Abstract: The attaching device for bag in which a pair of mouthpieces made of synthetic resin material is provided, and said pair of mouthpieces is provided with an engaging member formed by providing a stationary portion and a slope portion of a predetermined angle in the upper part of the stationary portion integrally by extending and projecting thereof and connecting a pawl portion integrally and foldably by means of a thin connecting portion, and a base plate formed by integrally projecting an edge portion formed with a groove portion on the lower part of the front surface of a long plate of a predetermined length, and said stationary portion is fixed to a reverse surface of the base plate and the tip of the pawl portion is engaged on the groove of the base plate, and when the stationary portion of the mouthpiece is urged against the other mouthpiece from the outside, the mutual slope portions extend and the pawl portion is separated from the groove portion of the base plate, and openable levers that are resilientl