To provide high density printing of a thermal head, to improve an image unevenness, and to further improve color tone by incorporating substance to be melted by heating and filler fine particles in a color developing material containing cobalt complex, aromatic dialdehyde and initiator.

The image forming material of color developing material containing cobalt complex, aromatic dialdehyde and initiator comprises substance to be melted by heating and at least one type filler fine particles. As the preferably used complex, a complex represented by a formula is used. In the formula, Lig is coordination compound for forming complex with cobalt, one of which indicates NH3 or primary amine, p indicates integer of 1 to 6, and when the p is the integer of 2 to 6, a plurality of the Lig may be the same or different. X indicates paired anions such as benzylate or acetate, q indicates 1, 2 or 3. As the aromatic dialdehyde, o-phthalaldehyde is used.