The Drabkin energy filter could be used as a fast chopper for pulsed neutrons by varying the resonant wavelength λ* fast in such a way that the resonant wavelength matches the neutron wavelength at a small time region. The faster λ* were varied, the smaller time window we could obtain. Neutron pulses could be sharpened much more effectively for a small wavelength band than in the wide-band pulse-shaping mode in which λ* is varied in synchronization with the time-of-flight from the moderator. In our preliminary test of a chopper mode with a prototype Drabkin energy filter, time window of 0.21 ms was obtained while it was 1.08 ms with the filter-driven stationary. Narrower time window could have been obtained if we had more beam time and it could also be narrowed with higher-resolution Drabkin filters.