  Mass segregation in a star cluster is studied in an analytical manner. We
consider a two-component cluster, which consists of two types of stars with
different masses. Plummer's model is used for the initial condition. We trace
the overall behaviors of the probability distribution functions of the two
components and obtain the timescale of mass segregation as a simple function of
the cluster parameters. The result is used to discuss the origin of a black
hole with mass of > 1000 M(sun) found in the starburst galaxy M82.
