A boiler steam amount measuring method for continuously measuring a temporal change in an amount of steam from a steam boiler, includes: first measuring a differential pressure between a pressure at a first detection position that is a predetermined position in a can body of the steam boiler or a steam outflow path, and a pressure at a second detection position in the steam outflow path separated from the first detection position toward a downstream side; first calculating a pressure loss coefficient based on the differential pressure measured by flowing a predetermined flow rate of steam or fluid instead of steam into the steam outflow path, and the predetermined flow rate; and second calculating continuously the amount of steam based on the differential pressure measured in the first measuring and the pressure loss coefficient calculated in the first calculating, and outputting the calculated amount of steam as a measurement value.