A novel diacetylene derivative, N,N′-bis[5-(3-tolylaminocarbonyloxy)-1,3-pentadiynyl]-N,N′-diphenyl-1,4-phenylenediamine (1), was prepared, where two diacetylene groups were connected by a 1,4-phenylenediamine moiety. The molecules stacked one-dimensionally and showed solid-state-polymerization reactivity above 80 °C. The decay of the monomers at 100 °C proceeded gradually without a marked induction period and was fully completed after 400 h. The obtained polymer was a crystalline solid judged by powder X-ray diffraction (PXRD) patterns and SEM imaging. The conjugated π-system of the obtained polymer was classified as a two-legged conjugated ladder. The polymer showed a broad absorption from the visible to the near IR region, indicating a decrease in the optical band gap of ca. 1.0 eV, because of the expansion of the π-conjugated system from a one-dimensional system to a ladder. The ladder polymer showed high conductivity after I₂ doping from σ_293K = 5 × 10⁻¹² S cm⁻¹ to 1.2 × 10⁻⁵ S cm⁻¹. The conductivity depended heavily on the pressure of iodine and reached σ_373K = 2.3 × 10⁻¹ S cm⁻¹. The activation energy of the ladder polymer was also estimated as 360 meV.