Provided are a bag mouth opening method and apparatus capable of surely and stably opening into a desired shape the mouth of a spout bag having a spout attached to an upper edge-side portion thereof. A spout tilting relative to a bag is corrected into a non-tilting position and pinched with spout pinching members opened and closed by a first air cylinder of a spout holding device. The first air cylinder is held in an initial position in the width direction of the bag by a second air cylinder through a quadric link comprising a connecting rod mounting plate, a connecting rod, a connecting plate, and a second support arm. The interior of the second air cylinder is opened to the atmospheric pressure in synchronism with the start of movement away from each other of a pair of suction cups of a bag mouth opening device and movement toward each other of a pair of gripping members gripping the laterally opposite side edges of the bag. Consequently, the first air cylinder and the pinching members attached thereto become free to move in the width direction of the bag and thus are moved through the bag in accordance with the movement of the gripping members.