This paper describes one-dimensional charge-transfer-type manganese porphyrin-tetracyanoethylene as the hardest molecule-based-magnet. The coercive forces of the magnets consisting of manganese(tetraphenylporphyrin) (MnTPP) and manganese(tetraethoxyphenylporphyrin) (MnTEtOPP) were 8000 Oe and 20 000 Oe at 2 K, respectively. The high coercive forces resulted from their metamagnetism, where both interchain ferromagnetic and antiferromagnetic interactions are balanced. Although the saturation magnetization of millimeter size crystal of MnTPP-TCNE is equal to that of powder, the initial magnetization processes are different owing to their domain structures. Almost consistent domain size of several decade nanometers was estimated from the powder X-ray diffraction and atomic force microscopy. Conventional relationships between the coercive force and the domain size hold true in the present molecule-based magnets.