In a substrate supporting apparatus of a surface potential measuring apparatus, a first fluid is ejected around a target region on an upper surface of a substrate from a circular-shaped first porous member of a first fluid ejection part and a second fluid is ejected onto a lower surface of the substrate from a circular-shaped second porous member of a second fluid ejection part which is opposite to the first fluid ejection part. The substrate can be supported and flattened between the first fluid ejection part and the second fluid ejection part. Also, it is possible to keep the distance between the substrate and the first porous member, with a simple construction. As a result, a probe can be positioned above a flatted target region with leaving a predetermined spacing, to perform measurement of a surface potential of the target region on the substrate with high accuracy.