A heat seal bar is mounted on and removed from a heater unit in a plastic bag making machine without difficulty, An insert is received and fixed to a receiving recess formed in the heat seal bar. A tension pin is supported by the heater unit, inserted into an adjacent recess, and then inserted into an axial groove through an axial opening. The tension pin is pulled and moved axially thereof by an actuator member at the heater unit until a head of the tension pin engages an inward protrusion of the insert. The tension pin and the insert cooperate to mount the heat seal bar on the heater unit.