Neutron structure analysis of poly(pyridobisimidazole) (PIPD) was carried out at 10, 100, 200, and 295 K. The crystal structure projected through the fiber axis was clarified. Two molecular chains pass through a rectangular unit cell with parameters a' = 12.85 Angstrom and b = 3.462 Angstrom (at 295 K), the plane group, pgg, PIPD assumes the statistical structure between two molecules with different orientation of pyridine ring. Two hydroxyl groups -OH extend outside from the chain axis. The temperature dependence of the cell parameters, a' and b, is very small, which is comparable to those of the rigid-rod polymers, poly(p-phenylenebenzobisoxazole) and poly(p-phenylenebenzobisthiazole), and is far smaller than the flexible polymers, polyethylene and poly(vinyl alcohol).