Sheet insert device, sheet processing apparatus and sheet processing method

According to one embodiment, a sheet insert device (100) includes a cassette (41) having a plurality of containing portions each of which contains a plurality of sheets in a vertically stacked state, a moving structure (47) to move the cassette so that the plurality of containing portions are arranged in turn at a pulling-out position, a grip arm (46) to grip the plurality of sheets contained in the containing portion arranged at the pulling-out position in the stacking direction, and a transferring structure which moves the grip arm, to pull out the plurality of sheets at the pulling-out position which are gripped by the grip arm from the containing portion, and transfers the plurality of sheets to a sheet loading part without changing the posture.