Grooves for receiving a movable loop forming plate are made in both front and back needle beds at the top ends thereof on the trick gap side, and movable loop forming plates are mounted in the grooves. A loop forming edge is provided at the top end of each movable loop forming plate, and the plate is energized by a spring in a direction that the plate moves away from the trick gap, and the plate is made to move forward or backward by a movable loop forming plate control means, around a fulcrum that is provided on the needle bed in the bottom of the trick gap.