component b can be easily produced by a process described in japanese patent laid open no 270859 1990 i e a process which comprises reacting an epihalohydrin with 2 mercaptoethanol and then with thiourea among the component c i e the aliphatic thiol compound having x hydroxyl groups x is an integer of 0 or 1 or more and y mercapto groups y is an integer of 1 or more wherein the number of the carbon atoms present between two hydroxyl groups two mercapto groups or a hydroxyl and mercapto groups which are most distant is 6 or less preferably 5 or less examples of the compounds in which the total number x y of the hydroxyl groups and the mercapto groups is 3 or more when the thiol compound has no ring structure in the molecule include monothioglycerol dithioglycerol trithioglycerol dimercaptopropanol 1 mercaptomethyl 1 1 dihydroxymethylpropane 1 4 dimercapto 2 3 hydroxybutane tetrakis mercaptomethyl methane 1 1 1 tris mercaptomethyl ethane and 1 1 1 tris mercaptomethyl propane examples of the aliphatic thiol compounds in which the total number x y of the hydroxyl groups and the mercapto groups is 2 or more when the thiol compound has a ring structure in the molecule include 2 5 dimercaptomethyl 1 4.