or more kinds of phosphonium compound selected from the group consisting of (a) a phosphonium compound wherein, in the above formula (I), at least one of R1 to R4 is a group represented by the formula (II), R5 to R7 are the same or different, and are a phenyl group, an optionally substituted benzyl group, a naphthylmethyl group, an optionally substituted diphenylmethyl group or an optionally subst