The invention provides a hot water supply system. The transient hot water supply system adopts two electronic valves (vapor regulation valve and a water level regulation valve) in order to make hot water discharged by a water supply pipe adjust by maintaining a certain temperature and comprises an attachment pressure supply part which can improve discharge pressure of condensate water during a process of discharging the condensate water in the water level groove. As a result, when usage load of the water supply causes acute change in flows, temperature deviation is reduced while the condensatewater in the liquid level groove is more smoothly discharged according to the discharge pressure of the condensate water.