A two-component multiblock copolymer with undecablock-two of them on both chain ends are long and nine of them are short-was successfully prepared by anionic polymerization using the six-step sequential monomer addition technique. Polymer components are polystyrene (S) and polyisoprene (I), its total molecular weight is 275K, and the overall S/I volume ratio is 0.70/0.30. Microphase-separated structure of the copolymer was observed by transmission electron microscopy and small-angle X-ray scattering, and it was confirmed that the copolymer forms a complex lamellar structure; its long period is 45 nm, which is composed of one thick lamellar domain formed by long polystyrene chains and I-S-I three thin lamellar domains, the length of the short period for I-S lamellae being about one-third of the longer period. This fact shows short block chains at the center favorably adopt a loop conformation over a bridge one. This unique lamellar structure having two length scales must be the first experimentally observed simple hierarchical structure for the block copolymer where the component polymers are connected by covalent bonds.