The invention provides a means for estimating a self-bias voltage under arbitrary etching conditions via a simple procedure. The present invention provides a method for measuring self-bias voltage of an etching apparatus comprising an electrostatic chuck mechanism 1 and 10 for chucking a sample 2, a mechanism 13 and 14 for supplying cooling gas 12 to a rear surface of the sample 2 and controlling the pressure thereof, and a means for measuring the relative force of electrostatic chuck of the sample based on the rear surface pressure control status of the sample 2 being processed, wherein the relative force of electrostatic chuck of the sample and the electrostatic chuck voltage corresponding to the force of electrostatic chuck are acquired based on the rear surface pressure control of the sample 2 when high-frequency bias power is applied to the sample 2 being processed, and the relative force of electrostatic chuck of the sample and the electrostatic chuck voltage corresponding to the force of electrostatic chuck are acquired based on the rear surface pressure control status of the sample when high-frequency bias power is not applied to the sample being processed, and the self-bias voltage is estimated using the acquired forces of electrostatic chuck and the electrostatic chuck voltages corresponding to the two statuses.