An optical splitter (1) includes a circulator (10) and a dynamic FBG (Fiber Bragg Grating) (20) which can achieve different reflection ratios corresponding to the size of a current in the dynamic FBG. The circulator has three ports. One port (P.sub.0) connects with an input optical fiber, one port (P.sub.3) connects with the dynamic FBG, and the third port (P.sub.2) connects with an output optical fiber. Light input from the circulator to the dynamic FBG and is dynamically split into two beams. One beam is output from an output port (P.sub.1) of the dynamic FBG, the other beam is reflected by the dynamic FBG and is output from the output port (P.sub.2) of the circulator.