Abstract: A disposable bag mouth opener is equipped to a bag (1) in a stage of charge and packing or in a stage of purchase by a consumer, the bag being sealed after charge of contents. This opener has a simple design comprising a bag attachment plate (2) and a holding device (3) for hold. The bag attachment plate (2) is provided with a mouth end (20) and a plate section (22) having characteristic of contact and separation and forming a cause of contact and separation of inner surfaces of the mouth end (20). The holding device is provided with an inner wall narrow portion (30) having a design which can hold and support the mouth end (20). The attachment plate (2) is attached by suitable means to the bag (1) at a section of bag mouth level of a projecting edge surface (10) previously provided or additionally provided on the bag (1) in the form of a projection.