Information flow type systems provide an elegant means to enforce confidentiality of programs. Using the proof assistant Isabelle/HOL, we have machine-checked a recent work of Boudol and Castellani~\cite{BC02:tcs}, which defines an information flow type system for a concurrent language with scheduling, and shows that typable programs are non-interferent. As a benefit of using a proof assistant, we are able to deal with a more general language than the one studied by Boudol and Castellani. The development constitutes to our best knowledge the first machine-checked account of non-interference for a concurrent language.