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 Vs=K[Vm(t1)+{1/(CmRm)}.intg..sub.0.sup.t1Vm(t)dt]
where K=(Cs+Cm)/Cs or K=Cm/Cs (if Cm>>Cs).