Abstract We present the results of mapping observations covering a large area of 1 square degree around W 40 and Serpens South carried out in the 12CO (J = 1–0), 13CO (J = 1–0), C18O (J = 1–0), CCS (JN = 87–76), and N2H+ (J = 1–0) emission lines with the 45 m Nobeyama Radio Telescope. W 40 is a blistered H ii region, and Serpens South is an infrared dark cloud accompanied by a young cluster. The relation between these two regions, which are separated by ∼20′ on the sky, has not been recognizable so far. We found the C18O emission is distributed smoothly throughout the W 40 and Serpens South regions, and that the two regions seem to be physically connected. We divided the C18O emission into four groups in terms of the spatial distributions around the H ii region which we call 5, 6, 7, and 8 km s−1 components according to their typical LSR velocity, and propose a three-dimensional model of the W 40 and Serpens South complex. We found two elliptical structures in the position–velocity diagrams, which can be explained as part of two expanding shells. One of the shells is small inner shell just around the H ii region, and the other is a large outer shell corresponding to the boundary of the H ii region. Dense gas associated with the young cluster of Serpens South is likely to be located at the surface of the outer shell, indicating that the natal clump of the young cluster is interacting with the outer shell being compressed by the expansion of the shell. We suggest that the expansion of the shell induced the formation of the young cluster.