  In recent numerical simulations \citep{matsubayashi07,lockmann08}, it has
been found that the eccentricity of supermassive black hole(SMBH) -
intermediate black hole(IMBH) binaries grows toward unity through interactions
with stellar background. This increase of eccentricity reduces the merging
timescale of the binary through the gravitational radiation to the value well
below the Hubble Time. It also gives the theoretical explanation of the
existence of eccentric binary such as that in OJ287 \citep{lehto96,
valtonen08}. In self-consistent N-body simulations, this increase of
eccentricity is always observed. On the other hand, the result of scattering
experiment between SMBH binaries and field stars \citep{quinlan96} indicated no
increase of eccentricity. This discrepancy leaves the high eccentricity of the
SMBH binaries in $N$-body simulations unexplained. Here we present a
stellar-dynamical mechanism that drives the increase of the eccentricity of an
SMBH binary with large mass ratio. There are two key processes involved. The
first one is the Kozai mechanism under non-axisymmetric potential, which
effectively randomizes the angular momenta of surrounding stars. The other is
the selective ejection of stars with prograde orbits. Through these two
mechanisms, field stars extract the orbital angular momentum of the SMBH
binary. Our proposed mechanism causes the increase in the eccentricity of most
of SMBH binaries, resulting in the rapid merger through gravitational wave
radiation. Our result has given a definite solution to the "last-parsec
problem".
