Abstract: A hierarchical message channel storage of a digital switching system which is connected to time division multiplex transmission lines includes at least a small-capacity high-speed memory and a large-capacity low-speed memory. A control section performs switching using the high-speed memory when a circuit switching call is received. However, when a packet switching call is received, the control section accesses the high-speed memory to temporarily store transmission data in the high-speed memory and performs switching for one of the output transmission lines or the control section accesses the high- and low-speed memories to temporarily store data in the low-speed memory through said high-speed memory so as to perform switching for one of said output transmission lines. In this case, the access cycles of the high-speed memory have circuit/packet switching call cycles and switching program cycles.