Bisphenol-phosphorus compound complexes represented by the following general formula (1) are disclosed (R.sup.1 to R.sup.4 represent hydrogen atom or a group that can be a substituent on a benzene ring; L represents --S-- group or a --CHR.sup.5 -- group where R.sup.5 represents hydrogen atom or an alkyl group; and R.sup.6 to R.sup.8 represent an alkyl group, an aryl group, a heterocyclic group etc.). The present invention provides reducing agents that can realize thermally processed image recording materials that can provide sufficient image density at a practical reaction temperature (specifically 100-140.degree. C.) and within a practical reaction time (specifically 1-30 seconds), and can sufficiently suppress the coloration of white portions when the materials are stored in a dark place after the development.