The electron spin density distribution at hydrogen atoms of 4-hydroxyimino-2,2,6,6-tetramethylpiperidin-1-yloxyl (4-hydroxyimino-TEMPO), which has recently been shown to act as a molecular ferromagnet at low temperature, was determined in the crystalline phase on the basis of the temperature dependence of the Fermi contact shifts in the magic angle spinning deuterium NMR spectrum to elucidate the mechanism of intermolecular magnetic interaction. There are two kinds of close contacts among neighboring radical molecules in the crystalline phase. An axial methyl hydrogen atom locates near a neighboring N-O radical group, and the hydroxyl group undergoes hydrogen bonding with another neighboring N-O radical group. The plus and minus signs of the observed hyperfine coupling constants A(D) of methyl deuteriums indicate that there are two different mechanisms for the electron spin density distribution. Equatorial CD3 groups show negative coupling constants (A(D) = -0.24 MHz) induced by an intramolecular spin polarization mechanism, whereas the positive hyperfine coupling constants (A(D) = +0.12 MHz) of axial CD3 groups indicate that a single occupied MO spreads out partly toward the axial CD3 groups by hyperconjugation. The small positive hyperfine coupling constant of the axial methyl group is due to averaging of the one positive and two negative values of the three deuterium atoms of the methyl group caused by rapid rotation. The intermolecular magnetic interaction through the axial methyl group seems to be sensitive to the orientation of the methyl group and can be ferromagnetic in a crystal of 4-hydroxyimino-TEMPO. The large negative hyperfine coupling constant (A(D) = -0.45 MHz) observed for the NOD group strongly implies that hydrogen bonding mediates the intermolecular magnetic interaction in the crystalline phase. The present experimental results and molecular orbital calculations indicate that ferromagnetic interaction exists in the hydrogen-bonded chains running along the crystallographic c axis, and that the two chains can be considered to be coupled ferromagnetically through the axial methyl groups to form a double chain. In the directions of the crystallographic a and b axes, weak ferromagnetic interactions are expected on the basis of the measured spin density distributions of the deuterium atoms of the equatorial methyl and the methylene groups which participate in interchain contacts in the crystallographic a-b plane. The H-1 and D-2 NMR spectra measured for solutions demonstrated that the spin density distribution of the radical molecule changes dramatically between the solution and crystalline phases.