The sulfonium or oxosulfonium complex of the formula (1) can be synthesized by reacting a sulfonium or oxosulfonium complex of the formula (2), ##STR3## wherein R1 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 R2 and R3 is independently any one of