The microwave introducing mechanism includes an antenna unit having a planar antenna radiating a microwave into a chamber; a tuner for performing impedance matching; and a heat dissipation device for dissipating a heat from the antenna unit. The tuner has a tuner main body including a tubular outer conductor and a tubular inner conductor to serve as a part of a microwave transmission line; slugs provided between the outer conductor and the inner conductor to be movable along a longitudinal direction of the inner conductor; and a driving device for moving the slugs. The heat dissipation device has a heat pipe configured to transfer the heat of the antenna unit from its heat input end to its heat dissipation end.