We report the magnetic properties of a novel organic polymer. The material gives the large saturation magnetization (0.15G/g) at room temperature. Electron spin resonance (ESR) revealed a ferrimagnetic character, i. e., the existence of "exchange resonance branch" with extremely large g-value. The new organic polymer with large saturation magnetization can be stoichiometrically and reproducibly synthesized under ambient conditions and expected to be useful for practical applications.