PURPOSE: To enhance photographic sensitivity and emulsion efficiency in converting photons into a developable latent image by forming the emulsion sensitized on the surface with the heavy metal compound and the thiourea compound.

CONSTITUTION: A combination of the heavy metal compound and the thiourea compound is used as a chemical sensitizer, and as the heavy metal compound, potassium hexachloroiridate is preferable, and as the thiourea compound, 3- acetoamidophenylthiourea is preferable. As the emulsion to be sensitized, the one of silver chlorobromide having a silver chloride content of ≥90mol% and cubic crystals and its edge length of 0.1-2.0μm is preferable, thus, permitting the obtained emulsion to prevent undesirable rise of contrast in the sensitization treatment, and photographic sensitivity accompanied by good contrast to be enhanced without adversely affecting photographic characteristics.