Abstract. Let R be a Dedekind domain, G an affine flat R-group scheme, and B a flat R-algebra on which G acts. Let A → B G be an R-algebra map. Assume that A is Noetherian. We show that if the induced map K ⊗ A → (K ⊗ B) K⊗G is an isomorphism for any algebraically closed field K which is an R-algebra, then S ⊗ A → (S ⊗ B) S⊗G is an isomorphism for any R-algebra S. §1. IntroductionIn this paper, we prove the following.Theorem 1. Let R be a Dedekind domain, G an affine flat R-group scheme, and M an R-flat G-module. Let A be a Noetherian R-algebra, and V a finitely generated A-module.K⊗G is an isomorphism for any algebraically closed field K which is an R-algebra, then the canonical map ϕ S : S ⊗ V → (S ⊗ M ) S⊗G is an isomorphism for any R-algebra S.As a corollary, we have the following.Corollary 2. Let R be a Dedekind domain, G an affine flat R-group scheme, and B a flat R-algebra on which G-acts. Let A be a Noetherian R-algebra, and ϕ : A → B G an R-algebra map. If the induced map ϕ K : K ⊗ A → (K ⊗ B) K⊗G is an isomorphism for any algebraically closed field K which is an R-algebra, then the canonical map ϕ S : S ⊗ A → (S ⊗ B) S⊗G is an isomorphism for any R-algebra S.So we may work only over algebraically closed field instead of general commutative ring, once we know that the action and the candidate of the generator and the relation of the invariant subring are given over a Dedekind domain (e.g., Z), and the group scheme in problem is flat over the Dedekind domain.