Coassociative submanifolds are 4-dimensional calibrated submanifolds in G2-manifolds. In this paper, we construct explicit examples of coassociative submanifolds in Λ 2 − S 4 , which is the complete G2-manifold constructed by Bryant and Salamon. Classifying the Lie groups which have 3-or 4-dimensional orbits, we show that the only homogeneous coassociative submanifold is the zero section of Λ 2 − S 4 up to the automorphisms and construct many cohomogeneity one examples explicitly. In particular, we obtain examples of non-compact coassociative submanifolds with conical singularities and their desingularizations.We give a summary about real irreducible representations in [5,18].Definition A.1. Let G be a compact Lie group and (V, ρ) be a C-irreducible representation of G. We call (V, ρ) self-conjugate if V has a conjugate linear map J on V satisfyingThis map is called a structure map. A self-conjugate representation (V, ρ) is said to be of index ±1 if J 2 = ±1.Proposition A.2. Let (V, ρ) be a C-irreducible representation of G. Then one of the following is satisfied.1. (V, ρ) is a self-conjugate representation of index 1. In this case, (V, ρ) is a complexification of a real representation. , J J 0 for (B.2),