The charging voltage measuring device includes a measuring electrode for forming
an electrostatic capacity Cs with a substrate disposed on a substrate holding unit,
a measuring capacitor, which has an electrostatic capacity Cm, being connected
between the measuring electrode and a ground potential portion, and, a voltage
measuring unit for measuring a measuring voltage Vm across the measuring capacitor,
and a calculating unit. The calculating unit 22 calculates the charging
voltage Vs on the surface of the substrate at time t1 in accordance with
the following numerical expression on the basis of the measuring voltage Vm(t1)
at time t1, an inverse K of a voltage dividing ratio and a resistance value
Rm of a resistor disposed in parallel to the measuring capacitor 18, when
the measurement time is t1.
where K=(Cs+Cm)/Cs or K=Cm/Cs (if CmCs).