The western Sea of Japan is a region in which mesoscale cyclones are frequently observed as cold polar air breaks out over the sea. In this region, mesoscale cyclones occasionally develop along a convergence zone that forms on the lee side of the mountains north of the Korean Peninsula. A meso-α-scale cyclone (MαC), in which two meso-β-scale cyclones (MβCs) were embedded, was observed in the western Sea of Japan on 23 January 1990. This structure is referred to as a “multi-scale structure” of a mesoscale cyclone. A simulation experiment and a number of sensitivity experiments with respect to several forcing factors were performed to elucidate the development mechanism of mesoscale cyclones. Stably stratified air flows around the mountains north of the Korean Peninsula and the convergence zone forms on the lee side of the mountains. Large amounts of sensible heat and latent heat are supplied from the sea to the atmosphere. The diabatic heating due to condensation and vertical diffusion of the sensible heat and horizontal advection of the potential temperature θ are almost in balance with the negative vertical advection of θ. This results in an intense upward motion in the convergence zone. Since baroclinicity is intense in this region, an upward transfer of the horizontal momentum around the convergence zone intensifies the upper-level (800∼600 hPa) divergence. Consequently, the surface pressure decreases, and the MαC develops. The upward motion and vorticity are concentrated mostly in the MβCs, which are considered to be the cores of the MαC. On the other hand, the MαC provides an environment for the formation and development of the MβCs and affects their movements.