The molecular conformation of a block polymer chain in a microphase-separated domain space (a confined space) was studied by small-angle neutron scattering (SANS) with a deuterium labeling technique. The samples studied were polystyrene-polyisoprene diblock polymers, and they have a morphology of highly oriented alternating lamellar microdomains composed of polystyrene (PS) and polyisoprene (PI) in bulk when cast from dilute solutions in toluene. Small-angle X-ray scattering (SAXS) measurements were conducted on the same specimens used for SANS in order to separate the scattering arising from a single chain P(q) and that arising from the microdomain structure S(q), Components of the radius of gyration of the single deuterated PS chain in PS lamellae, parallel (Rgx or Rgy) or perpendicular (Rgz) to the interfaces between the two microdomains, were determined with the high-concentration labeling technique of SANS coupled with the SAXS technique. The results indicated that (i) the chain is expanded normal to the interfaces, giving rise to Rgz≃1.6Rgx, (ii) the chain is contracted parallel to the interfaces, giving rise to Rgx ≃0.7Rgx, 0 (Rgx, 0 being a component of the radius of gyration of the corresponding unperturbed chain), and (iii) the longitudinal expansion is compensated by the lateral contraction, giving rise to an overall radius of gyration Rg nearly equal to or slightly less than Rg0 for the unperturbed chain. Conclusion (iii) does not mean at all that the chains in domain space are unperturbed but rather that they are strongly perturbed. The lateral contraction was proposed to be the consequence of the repulsive potential between the centers of block chains which are located in narrow interfacial regions (i.e., essentially in the two-dimensional space). A residual “memory” of the repulsion in the bulk block polymer could be a consequence of the two-dimensionality of the space available to chemical junctions of the block polymers and/or an effect of repulsive potential (which existed in the polymer solution with a good solvent) being “locked-in” at high polymer concentrations.