A gas flow rate detecting method comprising the steps of: determining in each of a plurality of successive second period intervals (ΔT), whether or not a particular gas instrument is activated, based on a gas flow rate obtained by a gas flow rate obtaining unit in each of a plurality of successive first period intervals (Δt), wherein each of the plurality of second period intervals (ΔT) includes the plurality of first period intervals (Δt); in a case where it is determined that the particular gas instrument is activated in a certain second period interval (ΔT), then calculating a total usage gas flow rate of the particular gas instrument, which is an integrated value of a usage gas flow rate of the particular gas instrument, based on the gas flow rate obtained by the gas flow rate obtaining unit, within the certain second period interval (ΔT), as to which it is determined that the particular gas instrument is activated, based on a change amount of the gas flow rate obtained by the gas flow rate obtaining unit within the certain second period interval (ΔT) in which the particular gas instrument is activated, and a usage gas flow rate of the particular gas instrument which is updated just before the certain second period interval (ΔT) in which the particular gas instrument is activated and setting a counter to 0; in a case where it is determined that the particular gas instrument is not activated in the certain second period interval (ΔT), then non-updating the usage gas flow rate of the particular gas instrument within the certain second period interval (ΔT) in which the particular gas instrument is not activated and adding 1 to the counter, until it is determined that the particular gas instrument is activated; and after determining that the particular gas instrument is activated again, then calculating a total usage gas flow rate of the particular gas instrument in a certain second period interval (ΔT) in which particular gas instrument is not activated, as to which it is determined that the particular gas instrument is not activated, by deeming the usage gas flow rate of the particular gas instrument which is updated just before it is determined that the particular gas instrument is not activated, as the usage gas flow rate of the particular gas instrument within the certain second period interval (ΔT) in which the particular gas instrument is not activated. determining in each of a plurality of successive second period intervals (ΔT), whether or not a particular gas instrument is activated, based on a gas flow rate obtained by a gas flow rate obtaining unit in each of a plurality of successive first period intervals (Δt), wherein each of the plurality of second period intervals (ΔT) includes the plurality of first period intervals (Δt); in a case where it is determined that the particular gas instrument is activated in a certain second period interval (ΔT), then calculating a total usage gas flow rate of the particular gas instrument, which is an integrated value of a usage gas flow rate of the particular gas instrument, based on the gas flow rate obtained by the gas flow rate obtaining unit, within the certain second period interval (ΔT), as to which it is determined that the particular gas instrument is activated, based on a change amount of the gas flow rate obtained by the gas flow rate obtaining unit within the certain second period interval (ΔT) in which the particular gas instrument is activated, and a usage gas flow rate of the particular gas instrument which is updated just before the certain second period interval (ΔT) in which the particular gas instrument is activated and setting a counter to 0; in a case where it is determined that the particular gas instrument is not activated in the certain second period interval (ΔT), then non-updating the usage gas flow rate of the particular gas instrument within the certain second period interval (ΔT) in which the particular gas instrument is not activated and adding 1 to the counter, until it is determined that the particular gas instrument is activated; and after determining that the particular gas instrument is activated again, then calculating a total usage gas flow rate of the particular gas instrument in a certain second period interval (ΔT) in which particular gas instrument is not activated, as to which it is determined that the particular gas instrument is not activated, by deeming the usage gas flow rate of the particular gas instrument which is updated just before it is determined that the particular gas instrument is not activated, as the usage gas flow rate of the particular gas instrument within the certain second period interval (ΔT) in which the particular gas instrument is not activated.