The present invention provides a novel indole carboxylic ester trimer which is characterized by being represented by the following general formula (1): ##STR00001## wherein R represents a straight or branched chain alkyl group having 1-6 carbon atoms; n is an integer of 1-4; X.sup.a- represents at least one anion selected from a group of anions having a valency of 1-3 consisting of chloride ion, bromide ion, iodide ion, fluoride ion, nitrate ion, sulfate ion, hydrogen sulfate ion, phosphate ion, borofluoride ion, perchlorate ion, thiocyanate ion, acetic acid ion, propionic acid ion, methanesulfonic acid ion, p-toluenesulfonic acid ion, trifluoroacetic acid ion and trifluoromethanesulfonic acid ion; a represents the ionic valence number of X and is an integer of 1-3; and m is 0-0.5.