Heat sensitive ink sheet and image forming method

A heat sensitive ink sheet having a support sheet and a heat sensitive ink layer having a thickness of 0.2 to 0.6 .mu.m which is formed of a heat sensitive ink material comprising 30 to 70 weight % of colored pigment, 25 to 65 weight % of amorphous organic polymer having a softening point of 40.degree. to 150.degree. C., and 0.1 to 20 weight % of a nitrogen-containing compound which comprises at least one compound selected from the group consisting of an amide compound having the formula (I):

• ##STR8## in which R.sup.1 represents an alkyl group of 8 to 24 carbon atoms, an alkoxyalkyl group of 8 to 24 carbon atoms, an alkyl group of 8 to 24 carbon atoms having a hydroxyl group, or an alkoxyalkyl group of 8 to 24 carbon atoms having a hydroxyl group, and each of R.sup.2 and R.sup.3 independently represents a hydrogen atom, an alkyl group of 1 to 12 carbon atoms, an alkoxyalkyl group of 1 to 12 carbon atoms, an alkyl group of 1 to 12 carbon atoms having a hydroxyl group, or an alkoxyalkyl group of 1 to 12 carbon atoms having a hydroxyl group, provided that R.sup.1 is not the alkyl group in the case that R.sup.2 and R.sup.3 both represent a hydrogen atom;
• a quaternary ammonium salt having the formula (II):
• ##STR9## in which R.sup.4 represents an alkyl group having 1 to 18 carbon atoms or an aryl group of 6 to 18 carbon atoms, each of R.sup.5, R.sup.6 and R.sup.7 independently represents a hydrogen atom, a hydroxyl group, an alkyl group of 1 to 18 carbon atoms, or an aryl group of 6 to 18 carbon atoms, and X.sub.1 represents a monovalent anion;
• and
• a quaternary ammonium salt having the formula (III):
• ##STR10## in which each of R.sup.8, R.sup.9, R.sup.10, R.sup.11, R.sup.12 and R.sup.13 independently represents a hydrogen atom, a hydroxyl group, an alkyl group of 1 to 18 carbon atoms or an aryl group of 6 to 18 carbon atoms, R.sup.14 represents an alkylene group of 1 to 12 carbon atoms, and X.sub.2 represents a monovalent anion.