Abstract: A pneumatic steel bar binding machine mechanism to bind two or more column-shaped steel bars together, or specifically, a machine to bind the steel frames inside the concrete framework in construction works, characterized in a pneumatic mechanism and a spiral twister that press steel staples in the shape of reversed-U onto the steel bars to be bound, before the spiral twister is driven to twist the two ends of the steel staple into a twisted rope, to secure the steel bars.