Disclosed is a variable lightwave functional circuit comprising: an applying unit for applying lateral pressure; a polarization maintaining optical fiber having a polarization axis, for inducing a polarization mode coupling by applying the lateral pressure to a predetermined position by said applying unit; and a branching/coupling device having a first port to a fourth port, for branching laser light inputted from the first port to both said second port and said third port, and also for projecting laser light inputted from said second port to the fourth port; wherein one end of said polarization maintaining optical fiber is connected to one of said second port and said third port in such a manner that the polarization axis is inclined at a predetermined angle to an axial direction parallel to a branching plane of said branching/coupling device.