provided that at least one of R1 to R4 is a group represented by the formula (II), or any one of R1 to R4 and any one of R5???R7 may be combined together to form a lower alkylene group which forms a heterocyclic ring containing two phosphorus atoms, and X is as defined in the formula (II); (2) The epoxy resin composition according to the above-mentioned (1) wherein the phosphonium compound is one