Abstract: A static pressure non-contact type mechanical seal that exhibits and maintains a high sealing function or performance for a prolonged period even when sealing gases containing foreign matter such as oil. A labyrinth seal (7) comprising a plurality of annular grooves (71) and annular plates (72) extending into the grooves is provided, the labyrinth seal partitioning a region C on the outside diameter side of the seal faces (31, 41) in the seal case (2) from the inside region A of a gas treatment machine (1). The seal case is provided with a return passage (8) communicating with the bottom portions of the annular grooves 71 and opening into the inside region A of the machine. The seal case also includes a purge gas feed passage for supplying purge gas to the region C on the outside diameter side of the seal faces, the purge gas pressure being higher than that in the inside region.