Body elongation is a general feature of development. Postembryonically, the body needs to be framed and protected by extracellular materials, such as the skeleton, the skin and the shell, which have greater strength than cells. Thus, body elongation after embryogenesis must be reconciled with those rigid extracellular materials. Here we show that the exoskeleton (cuticle) coating the Drosophila larval body has a mechanical property to expand less efficiently along the body circumference than along the anteroposterior axis. This "corset" property of the cuticle directs a change in body shape during body growth from a relatively round shape to an elongated one. Furthermore, the corset property depends on the functions of Cuticular protein 11 A and Tubby, protein components of a sub-surface layer of the larval cuticle. Thus, constructing a stretchable cuticle and supplying it with components that confer circumferential stiffness is the fly's strategy for executing postembryonic body elongation.