Isidor Marcellus Ganneval (whose well-known family name “Canevale”does not occur in contemporary sources) went to Vienna together with Servandoni. He was commissioned with the building of Vác Cathedral by Bishop Count Christoph Migazzi in 1761, by changing the plans made by Franz Anton Pilgram at the time of Bishop Károly Eszterházy. Keeping the foundations already laid, Ganneval altered the aspect of the church building in the spirit of French Neoclassicism both in the service of Catholic ideas of Rome and in the sense of Winckelmann's thoughts. The second part of the study is devoted to the motif of the triumphal arch in Ganneval's work. That in Vác, which was built by Bishop Migazzi for the visit of Empress Maria Theresa to the city, is in close relationship with Migazzi's patronage. The Author attributes also a triumphal arch in Heusenstamm, the estate of Count E. E. von Schönborn (near Frankfurt a. M.) to Ganneval. Further, the entrance gate to the Augarten in Vienna and the impact of Ganneval's creations in Hungary and Austria are studied.