There is disclosed a substrate processing apparatus which is capable of detecting whether a substrate, such as a wafer, supported by a fluid is properly present in a predetermined processing position or not. The substrate processing apparatus includes at least one distance sensor configured to measure a distance between a scrubber and a hydrostatic support structure; and a processing controller configured to calculate a gap between the hydrostatic support structure and the surface of the substrate from a measured value of the distance and determine whether the gap falls within a predetermined range or not.