Provided is a bag making apparatus with a simple configuration that: prevents fluctuations in tension, slippage of seal positions, and meandering or skewing of a web of film; allows the setting of precise initial conditions; reduces or makes unnecessary monitoring, and time and structures for adjustment; allows increased feed rate; and improves production efficiency. The bag making apparatus for packaging containers comprises: a tension control unit for controlling the continuous feed rate according to the values detected by a tension sensor of a tension detection roll; an optical detection unit for simultaneously recognizing multiple marks provided on the surface of the web of film at the respective positions thereof; a skewing correction unit; and a seal position correction unit for adjusting the positions of multiple sealing units in the feed direction.