A novel spin filter consisting of a triple-barrier resonant tunneling system in the form F/I/N/I/F/I/F is proposed, where F, I, and N represent a ferromagnetic material, an insulator, and a nonmagnetic material, respectively. The spin-dependent tunneling current in the triple-barrier resonant tunneling system is calculated theoretically on the basis of a Tsu-Esaki formula to investigate the output tunnel current polarization. Detailed calculations using the GaMnAs/AlAs/GaAs material system show that the two clear split peaks originating from up- and down-spin holes appear in the current-voltage (I-V) curve due to spin splitting of the energy levels formed in the ferromagnetic quantum well. The polarization can reach more than 98% at the peak positions in the I-V curve.