  We are developing a neutron polarizer with polarized $^3$He gas, referred to
as a $^3$He spin filter, based on the Spin Exchange Optical Pumping (SEOP) for
polarized neutron scattering experiments at Materials and Life Science
Experimental Facility (MLF) of Japan Proton Accelerator Research Complex
(J-PARC). A $^3$He gas-filling station was constructed at J-PARC, and several
$^3$He cells with long spin relaxation times have been fabricated using the
gas-filling station. A laboratory has been prepared in the MLF beam hall for
polarizing $^3$He cells, and compact pumping systems with laser powers of 30~W
and 110~W, which can be installed onto a neutron beamline, have been developed.
A $^3$He polarization of 85% was achieved at a neutron beamline by using the
pumping system with the 110~W laser. Recently, the first user experiment
utilizing the $^3$He spin filter was conducted, and there have been several
more since then. The development and utilization of $^3$He spin filters at MLF
of J-PARC are reported.
