  Motivated from the flat-band ferromagnetism conceived theoretically for a
single chain of five membered rings (polyaminotriazole) by Arita {\it et al.},
[Phys. Rev. Lett. {\bf 88}, 127202 (2002)], we have studied whether the
magnetism can indeed occur as a bulk, i.e., in a three-dimensional crystal of
the polymer, by means of the spin density functional calculation. We find that
the intra-chain ferromagnetism is robust against crystallization as far as the
flat band is made half-filled. We have further investigated the actual crystal
doped with various compounds, where HF$_2$ is shown to put the system close to
the bulk ferromagnetism while stronger anions such as BF$_4$ or PF$_6$ should
be promising.
