The invention provides a substrate floating device and a substrate floating quantity measuring method which can constantly and accurately calculate and control the floating quantity of a substrate. The substrate floating device (1) enables a substrate to float and comprises a floating unit (2) and an analyzing device (21). The floating unit comprises an opposite surface (14) facing the back surface of the floating substrate (W) and the substrate floats to be above the opposite surface. The analyzing device calculates the floating quantity of the substrate floating from the opposite surface. A light reflection type height measurement device (12) used for reflecting light to the substrate floating on the floating unit is arranged below the opposite surface. The floating unit comprises a through hole (15) and a plate-shaped datum plate (13), wherein the through hole enables light emitted from the height measurement device and light reflected from the substrate and emitted to the height measurement device to pass through and the datum plate is arranged on the path of light emitted to the substrate from the height measurement device via the through hole and reflects and transmits a part of the light.