A steam boiler having a steam accumulator connected between the boiler and a user, a flow meter provided on the inlet side of the steam accumulator, and a pressure detector provided on the steam accumulator for detecting the internal pressure thereof, wherein the steam boiler is arranged to detect by the flow meter the steam flow rate on the inlet side of the steam accumulator, which is varied in the fashion of following the mean value of steam load, to detect the internal pressure of the steam accumulator by the pressure detector, and to calculate the steam load on the outlet side of the steam accumulator by a steam load detector on the basis of signals of detected steam flow rate and pressure variation.