Abstract: A sulfonium complex or oxosulfonium complex of the formula (1), ##STR1## wherein R.sup.1 is a benzyl group, a substituted benzyl group, a phenacyl group, a substituted phenacyl group, an aryloxy group, a substituted aryloxy group, an alkenyl group or a substituted alkenyl group, each of R.sup.2 and R.sup.3 is independently any one of groups defined as R.sup.1, an alkyl group, a substituted alkyl group, an aryl group, a substituted aryl group, an aralkyl group, a substituted aralkyl group, an alkynyl group, a substituted alkynyl group, an alicyclic group, a substituted alicyclic group, an alkoxyl group, a substituted alkoxyl group, an alkylthio group, a substituted alkylthio group, an amino group or a substituted amino group, or R.sup.2 and R.sup.3 may bond together forming a ring structure, R.sup.4 is an oxygen atom or lone pair, and each of R.sup.5, R.sup.6, R.sup.7 and R.sup.