A pair of conductive rubber electrodes including measurement surfaces opposite to a surface of a dielectric layer of an electrostatic chuck as an objective of measurement, in which the measurement surfaces are arranged at an interval individually on the same plane, are provided. A direct-current power supply and an ammeter are connected to the pair of conductive rubber electrodes. The conductive rubber electrodes have resistance values equal to each other, and have a shape in which the measurement surfaces have areas equal to each other, in which volume resistivities are 1×105 Ω·cm or less, and hardness is within a range of 60 to 80 Hs in JIS-A hardness. An interval between the conductive rubber electrodes is six times or more a thickness of the dielectric layer of the electrostatic chuck as the objective of the measurement.