  We present a study on the evolution of the small scale velocity field in a
solar filament as it approaches to the eruption. The observation was carried
out by the Solar Dynamics Doppler Imager (SDDI) that was newly installed on the
Solar Magnetic Activity Research Telescope (SMART) at Hida Observatory. The
SDDI obtains a narrow-band full disk image of the sun at 73 channels from
H$\alpha$ - 9.0 \AA\ to H$\alpha$ + 9.0 \AA, allowing us to study the
line-of-sight (LOS) velocity of the filament before and during the eruption.
The observed filament is a quiescent filament that erupted on 2016 November 5.
We derived the LOS velocity at each pixel in the filament using the Becker's
cloud model, and made the histograms of the LOS velocity at each time. The
standard deviation of the LOS velocity distribution can be regarded as a
measure for the amplitude of the small scale motion in the filament. We found
that the standard deviation on the previous day of the eruption was mostly
constant around 2-3 km s$^{-1}$, and it slightly increased to 3-4 km s$^{-1}$
on the day of the eruption. It shows further increase with a rate of 1.1 m
s$^{-2}$ about three hours before eruption and again with a rate of 2.8 m
s$^{-2}$ about an hour before eruption. From this result we suggest the
increase in the amplitude of the small scale motions in a filament can be
regarded as a precursor of the eruption.
