We made direct force measurements by atomic force microscopy (AFM) to elucidate the elastic properties of a single polymer chain in a "semidilute" polymer brush in a good solvent (toluene in this study). A block copolymer brush comprised of low-polydispersity poly(methyl methacrylate)-b-poly-(4-vinylpyridine) (PMMA-b-P4VP) chains end-grafted on a silicon substrate was prepared by the surface-initiated atom transfer radical polymerization technique. The graft density of this brush (0.07 chians/nm(2)) is high enough to form a semidilute polymer brush. The introduction of a short P4VP block at the free chain ends of a PMMA brush made it possible to selectively pick up the chain ends and stretch the PMMA chains by an AFM tip. Upon retraction of the AFM tip, discrete multiple attraction peaks were observed, each of which was attributed to the detachment of a single graft chain from the AFM tip. Each elastic response curve could be well-fitted to the wormlike chain(WLC) model, giving an estimate of the contour length L-c of the related single chain. These analyses revealed that the elastic properties of a single polymer chain in the semidilute brush may be well-described by single-chain statistics such as the WLC model, at least in a highly stretched region, and that the adhesive force of the P4VP block on the AFM tip was strong enough to extend the graft chains up to about 90% of L-c. The distribution of L-c thus estimated was close to the chain length distribution of the PMMA block. This means that the length and length distribution of graft chains could be estimated by the AFM force analysis.