Bisphenol-phosphorus compound complexes represented by the following general formula (1) are disclosed (R1 to R4 represent hydrogen atom or a group that can be a substituent on a benzene ring; L represents —S— group or a —CHR5— group where R5 represents hydrogen atom or an alkyl group; and R6 to R8 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° 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.