In a semiconductor device manufacturing apparatus and a semiconductor device manufacturing method, a furnace tube port gas introducing pipe (9) for supplying gas to only one end portion of a furnace tube (2) is provided separately from a process gas introducing pipe (5) for supplying process gas into the furnace tube (2), and when wafers (4) are inserted into the furnace tube (2), an oxygen atmospheric layer (11) is formed only at the furnace tube port by oxygen gas or oxygen gas diluted with nitrogen gas which is supplied from the furnace tube port gas introducing pipe (9).