Heat-sensitive recording material

A heat-sensitive recording material having on a base sheet a heat-sensitive recording layer which comprises a colorless or pale colored basic chromogenic material and a color developer, characterized in that the recording layer comprises at least one fluoran derivative developing a black color and at least one azaphthalide derivative represented by the formula (I) as the basic chromogenic material, and at least one diphenylsulfone derivative represented by the formula (II) as the color developer, the amount of the azaphthalide derivative being within the range of 3 to 50% by weight based on the amount of the fluoran derivative:

• ##STR4## in which each of A and B is carbon or nitrogen atom, but at least one of them is nitrogen atom;
• each of R.sub.1, R.sub.2, R.sub.3 and R.sub.4 is hydrogen, alkyl, unsaturated alkyl, cycloalkyl, aryl, aralkyl or tetrahydrofurfuryl, the aryl and aralkyl may be substituted by at least one substituent selected from the group consisting of halogen, alkyl and alkoxyl, and R.sub.1 and R.sub.2, or R.sub.3 and R.sub.4 may form a ring with the neighboring nitrogen atom;
• each of X and Y is hydrogen, halogen, alkyl, alkoxyl or acyloxy, but both of X and Y are not hydrogen atom;
• ##STR5## in which each of R.sub.5 and R.sub.6 is C.sub.1 -C.sub.4 alkyl, C.sub.2 -C.sub.4 alkenyl, C.sub.1 -C.sub.4 alkoxyl, benzyloxy or halogen atom, m is 0, 1 or 2, n is an integer of 1 to 3 and each of p and q is 0, 1 or 2.