A load port apparatus connects a main opening of a wafer transportation container with a frame opening. The load port apparatus includes an installation part, a frame, and a flange clamp. The installation part has an installation table configured to install the wafer transportation container and move to and from the frame opening. The frame is upright from the installation part and has the frame opening. The flange clamp includes an engagement portion and a drive portion. The engagement portion is configured to be engaged with a flange surrounding an outer circumference of the main opening. The drive portion is configured to drive the engagement portion. The engagement portion is engaged from above or the side with a flange groove formed in the flange and opening radially outwardly.