Dimethylaminomethyl copper phthalocyanine and/or its derivative represented by the general formula, ##STR1## wherein CuPc represents a copper phthalocyanine residue, X represents a hydrogen atom, a halogen atom, the group --SO.sub.3 H or the group --CH.sub.2 OH, and m and n independently represent an integer of 1 to 4,and containing only small amounts of by-products is produced by a process comprising reacting aminomethyl copper phthalocyanine and/or its derivatives represented by the general formula ##STR2## wherein CuPc, X, m and n are as defined above, with formic acid and a formaldehyde-generating substance at a temperature of 130.degree. to 150.degree. C. under a pressure of 3 to 10 kg/cm.sup.2 (gauge).