The polishing process includes a first state where an eddy current sensor and a polishing target object do not face each other and a second state where the eddy current sensor and the polishing target object face each other. The method of correcting a film thickness measurement value includes obtaining a first measurement signal (Xout, Yout) output from the eddy current sensor in the first state (step S108), computing a correction value (ΔX, ΔY) on the basis of the obtained first measurement signal and a reference signal (Xsd, Ysd) set in advance, obtaining a second measurement signal (X, Y) output from the eddy current sensor in the second state (step S104), and correcting the obtained second measurement signal on the basis of the computed correction value while the polishing process is being performed (step S105).