Let E be an elliptic curve over Q, p an odd prime number and n a positive integer. In this article, we investigate the ideal class group Cl(Q(E[pn])) of the pn-division field Q(E[pn]) of E. We introduce a certain subgroup E(Q)ur,pn of E(Q) and study the p-adic valuation of the class number #Cl(Q(E[pn])).In addition, when n=1, we further study Cl(Q(E[p])) as a Gal(Q(E[p])/Q)-module. More precisely, we study the semi-simplification (Cl(Q(E[p]))⊗Zp)ss of Cl(Q(E[p]))⊗Zp as a Zp[Gal(Q(E[p])/Q)]-module. We obtain a lower bound of the multiplicity of the E[p]-component in the semi-simplification when E[p] is an irreducible Gal(Q(E[p])/Q)-module.