In the phase-shift measurement method, the distance light travels can be obtained via the phase difference between the reference and the measured signals. In this paper, a multiple-step phase demodulation method with time counting is proposed for measuring the phase difference, with resulting robustness against electric noise. When the phase differences are close to 2nπ (n = 0, 1, 2, ...]), 2π ambiguity problems commonly occur; here, a time-counting method using a 180°-shifted reference signal is proposed in order to resolve this problem.