A spin-polarized donor, BTBN, which is a dibromotetrathiafulvalene derivative containing a nitronyl nitroxide group in a cross-conjugated manner, was prepared. Upon hole injection from an electrode, the neutral microcrystals of BTBN exhibited nonlinear I-V characteristics that were interpreted in terms of the space-charge-limited conduction (SCLC) mechanism. Moreover, the resistance of BTBN decreased upon application of a magnetic field below 30 K and exhibited a giant negative magnetoresistance of (R(H) - R(0))/R(0) = -76% at 2 K under 5 T. These results show that the transport of carriers in the neutral unicomponent radical crystal can be controlled by the external magnetic field. These findings are important as a basis for developing molecule-based spin electronic devices.