PURPOSE: To improve an yield by reducing depositing possibility of dust to a substrate to be treated as compared with heretofore.

CONSTITUTION: A gas feeding piping 7 is connected to one side side wall and an exhaust piping 8 is connected to a side wall on the opposite side to this side wall in a load lock chamber 4 for carry-in and a load lock chamber 5 for carry-out. Gas fed from this gas feeding piping 7 is diffused by a rectifying plate 9 for cleaning by a filter 10 so as to send out from the almost whole surface of the side wall and perform exhaust from the opposing side wall through a rectifying plate 11 in order to form a laminar flow of purified gas almost at the level as the surface of a semiconductor wafer 2 inside the load lock chamber 4 for carry-in and the load lock chamber 5 for carry-out.