A phase difference detecting device includes a splitter for splitting laser beams into a first group which will travel along a first path and a second group which will travel along a second path, a beam selection/extraction unit for selecting, as reference light, one beam from the first group to allow it to pass therethrough, a path length changing unit for changing the length of the first path, a combining unit for combining the reference light and beams which construct the second group to produce interference light, and a detector for detecting the intensity of the interference light. The device changes the length of the first path using the path length changing unit to detect a path length which maximizes the intensity of the interference light for each of the beams which construct the second group, and determines a phase difference among the beams from the detected path length.