The present invention relates to a high efficiency vacuum boiler which can perform more smooth heating circulation by mitigating degree of vacuum through additional storage of condensate which is circle-circulated together with steam during a boiler operation procedure. The present invention provides the vacuum boiler comprising: a boiler body (10) where a heating medium is stored in a fixed level for heat exchange with a main steam pipe (P) comprised in a shape of surrounding the vicinity thereof; a distributor (20) for distributing the steam heated in the boiler body (10) to a number of places used (30); a return water unit (40) for returning the steam passing through the place used (30); a vacuum pump (50) to return the water to the boiler body (10) forcibly by applying vacuum force to the main steam pipe (P) connected to the return water unit (40); a condensate tank (60) which is connected between the vacuum pump (50) and the return water unit (40) in order to keep the condensate contained in the main steam pipe (P) separately; a buffering chamber (70) to supply the condensate to the boiler body (10) by controlling degree of internal vacuum through buffering and mounted in the condensate tank (60); a circulation pump (80) to increase supply ability of the condensate comprised in an auxiliary steam pipe (P′) connected to guide the condensate discharged from the buffering chamber (70) to the main steam pipe (P); and a control valve (81) to control amount of the condensate supplied comprised in the auxiliary steam pipe (P′).
