We present a formal proof of the classical Tarjan-1972 algorithm for finding strongly connected components in directed graphs. We use the Why3 system to express these proofs and fully check them by computer. The Why3-logic is a simple multi-sorted first-order logic augmented by inductive predicates. Furthermore it provides useful libraries for lists and sets. The Why3 system allows the description of programs in a Why3-ML programming language (a first-order programming language with ML syntax) and provides interfaces to various state-of-the-art automatic provers and to manual interactive proof-checkers (we use mainly Coq). We do not claim that this proof is new, although we could not find a formal proof of that algorithm in the literature. But one important point of our article is that our proof is here completely presented and human readable.