PURPOSE: To improve the extent of measuring accuracy by using only an area high in linearity, excepting the neighborhood of an apex, where an error is large, of a chopping signal so far used in the conventional phase difference measuring device.

CONSTITUTION: Reference interference light (a) is subjected to square-law detection by a photodiode 1, converted into a current signal, setting it down to a reference signal, while it is converted into a voltage signal by a converter 3, further converting it into a rectangular wave 5, and it is divided by a 1/2 divider 7, thereby synchronizing the signal with rising and falling, then it is divided by each of 1/2 dividers 9, 10, thus two rectangular wave signals shifted 1/4 cycle phase with each other are generated. After removing a DC component, they are set down to two chopping signal shifted as far as a 1/4 period by integrating circuits 13, 14, forming a composite signal 15, and fed to an arithmetic circuit 16. On the basis of the reference signal, a rectangular wave clock signal is formed via a current voltage converter circuit 4 and a comparator 6, and it is inputted into the arithmetic circuit 16, reading a voltage value of the composite signal in synchronization, then it is converted into a digital value, setting it down to a phase difference output, and fed to a DSP 19, through which a fringe internal phase difference is added to a difference of the fringe number counter by an up-down counter 18, calculating the phase difference.