To provide an automatic bag opening device which is capable of saving an installation space to a minimum, and reducing the remaining quantity constantly to a minimum even when the bag is continuously opened and the content is discharged.

An automatic bag opening device 1 comprises a frame 13, a bag carry-in mechanism 41 to carry a bag to a sensor 30 for sensing the carry-in bag stopping position on a bag mounting table 29, a carry-in bag constantly supporting mechanism 83 which also presses and supports the bag, a bag opening mechanism by a large number of abrasive grains fixed to an outer circumference of a rotary grinding and cutting body 93, a turning and oscillation actuator 43 to achieve the 'slapping action' for a shrunk bag so that the bag is turned around a turning and supporting shaft 15 together with the bag mounting table 29 to drop and discharge a raw material from a horizontally long, narrow cut opening, and the shrunk bag is turned together with the bag mounting table around the turning and supporting shaft, a shaking-off mechanism 109 to promote the discharge of the left raw material by the 'tapping action' on the shrunk bag by a short amplitude vibration generating means 105, and a control means to control the actions of these mechanisms.