A surface treatment apparatus in which a disk-like sample-holding plate is provided inside an enclosure constituting a cylindrical circumferential wall. A cylindrical portion in an upper portion of the enclosure constitutes a material fluid supplying channel, and a channel provided on the lateral side of the sample-holding plate in the enclosure and shaped spreading as it goes farther from the cylindrical portion constitutes a fluid discharge channel. The fluid discharge channel employs a parabola curve or the like in which the position of the upper end of the outmost circumference of the sample-holding plate is defined as a focus position and the position of the upper end of the outlet that is symmetrically opposite to the focus position is defined as a reference position.