The photobase generator of the invention is represented by the following formula 1: ##STR00001## wherein Ar, R, A.sup.+ and X.sup.- are as defined in the specification. Since the photobase generator of the formula 1 absorbs ultraviolet lights of relatively long wavelength and is photolyzed to generate a strong base efficiently, a composition containing the photobase generator and an episulfide compound is easily cured by polymerization under ultraviolet irradiation.