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 during continuous movement, and time and structures for adjustment; allows increased feed rate of the web of film; and improves production efficiency. The bag making apparatus (100) for packaging containers that seals prescribed areas of the web of film (F) comprises: a tension control unit (165) for controlling the continuous feed rate according to the values detected by a tension sensor (164) of a tension detection roll (163); an optical detection unit (140) for simultaneously recognizing multiple marks (M) provided on the surface of the web of film (F) at the respective positions thereof; a skewing correction unit (150); and a seal position correction unit (180) for adjusting the positions of multiple sealing units (171) in the feed direction.
