We have been developing a 3 He neutron spin filter for efficient utilization of pulsed neutrons at J-PARC MLF. We performed the demonstrative study of a polarized-neutron diffraction measurement of single crystal Fe with our developed 3 He spin filter as the polarizer at BL18 in J-PARC MLF. The observed flipping ratio of the Bragg peak was much lower than that was expected. Therefore, we have performed a depolarization experiment of neutrons transmitted through the sample at BL10. As the result, severe neutron depolarization was found to occur on the neutrons with corresponding wavelength when they transmit through the sample. By taking the depolarization effect into account, the observed low flipping ratio of the Bragg peak could be well understood.