Provided are a small-sized, low-cost, and easy-to-use laser optical path length difference detecting device, a laser optical path length difference detecting device, and a coherent optical coupling device. The laser optical path length difference detecting device detects an optical path length difference between propagation paths of a first laser beam (1) and a second laser beam (2), which are mutually coherent when the beams are propagated through two arbitrary optical paths of a plurality of laser beam optical paths. The laser optical path length difference detecting device is provided with an optical path length difference variable means (4) for changing the optical path length difference between the first laser beam (1) and the second laser beam (2), a wavefront inclination generating means (5) for inclining a wavefront of at least one of the first laser beam (1) and the second laser beam (2), and a two-dimensional detector (6) for detecting the interference light intensity distribution of the first laser beam (1) and the second laser beam (2) which have passed through the wavefront inclination generating means.