To provide a simple and effective system capable of providing the energy-saving effect and the heat island phenomenon restraining effect in a regional scale similarly to a regional cooling-heating system.

While maintaining the supply water temperature of heat source water in a specific range through years by a heat source facility 11 in a heat source plant 10, the heat source water is supplied as a heat radiating source in cooling operation and a heat gathering source in heating operation to a water heat source heat pump 20 of respective buildings 1. This regional heat source system has a cooling tower facility 31 and a heating source facility 32 as an auxiliary heat source facility 30 when necessary, by adopting a heat exchanger for radiating surplus heat and gathering shortage heat by giving and receiving heat with city sewage as the heat source facility. An air-cooled outdoor machine in an existing air heat source heat pump arranged in an existing building, is replaced with or remodeled into a water-cooled outdoor machine 21, and the heat source water is supplied to this machine.