PURPOSE: To enhance stability in manufacture and, especially, to reduce deterioration of performance during storage, and to enhance resistance to stagnation of a coating fluid and aging stability under high temperature and high humidity by adding a specified step for specifying a silver potential at the time of preparing the emulsion and a specified radical scavenger.

CONSTITUTION: The step of regulating the silver potential to the range of -200 to -50mV is included into the process of preparing the emulsion and the radical scavenger to be incorporated is represented by formula I or II in which each of R and R' is, independently, an alkyl or aryl group, and each of R1 and R2 is, independently, a hydroxyamino or hydroxy group or the like. The radical scavenger is designated as the compound capable of substantially decoloring carbionxyl (reducing light absorbance at 230nm, when a 0.05mmol/dm3 ethanol solution of the carbinoxyl are mixed with 2.5mmol/dm3 ethanol solution of the compond to be tested at ≤25°C by the stopped-flow method and change of the absorbance at 430nm with the lapse of time is measured.