PURPOSE: To enhance color formability and long-term preservability, by incorporating a water-insoluble binder in a layer for forming a lower layer and a biguanidine compound or an acidic salt thereof in at least one of a diazo layer and a coupler layer.

CONSTITUTION: In a diazo type thermal recording material having a heat-sensitive color forming layer having a structure formed by laminating a diazo layer and a coupler layer, a water-insoluble binder is incorporated in a layer for forming a lower layer and a biguanidine compound or an acidic salt thereof is incorporated in at least one of a diazo layer and a coupler layer. A representative biguanidine compound is shown by the formula wherein R1WR6 are each hydrogen, alkyl, cycloalkyl, hydroxy, alkoxy, an aromatic group or a heterocyclic group. By the addition of the biguanidine compound, thermal color forming reaction is remarkably promoted and the above-described laminar structure serves to keep long-term preservability.