To perform a heat treatment without using a liquid agent by using a specific development agent for heat development.

This is a development agent for heat development expressed by the formula. In the formula, W is-NR1R2,-OH, or-OM, R1 and R2 are respectively an alkyl or aryl group, and M in an alkaline metal atom or ammonium group. R3 is a hydrogen atom, halide atom, or monovalent substitutent, (n) is an integer from 1 to 3. When (n) is 2 or more, a plurality of R3 may be the same or be different. Z1 and Z2 are respectively a nitrogen atom or CR4, and one of them is nitrogen atom. X is an atom group required to form a five to six membered aromatic heterocylic ring with Z1 and Z2, and a carbon atom adjacent to them. A development agent expressed by the formula is desirably treated added as a salt. There is no limitation for acids to form a salt. Organic acid such as sulfuric acid, sulfurous acid, and hydrochloric acid can be used. Sulfate is desirable regarding preservability and handling.