A flow rate measuring apparatus comprising: a flow rate measurement unit that measures at given time intervals a flow rate of gas flowing through a passage and that outputs measurement results as flow rate data; a computing unit that calculates differential values from the flow rate data output from the flow rate measurement unit; an appliance determination unit that determines use of an appliance from the differential values computed by the computing unit; and an appliance flow rate calculation unit that calculates a usage flow rate for each appliance, characterized in that the appliance flow rate calculation unit calculates a usage flow rate for each appliance by switching a calculation method in accordance with a result of determination made by the appliance determination unit, the appliance flow rate calculation unit has a plurality of calculation processing units that calculate a flow rate in predetermined steps; and the switching of the calculation processing units is based on: the number of times of flow rate measurements performed by the flow rate measurement unit during a time from when a transition to a state of determination of an appliance is effected and to when processing leaves the state of determination, wherein the state of determination of the appliance starts when the appliance determination unit determines a gas appliance having started operation due to an amount of change of the flow rate of gas coming to a value other than zero and wherein the state of determination of the appliance ends either when the appliance determination unit determines the gas appliance having ended operation due to the amount of change of the flow rate of gas coming to zero again or when a predetermined time has elapsed from the transition to the state of determination of the appliance, wherein the predetermined time means the longest time among times that are required for the appliance determination unit to determine an appliance and that change from one gas appliance to another, and the number of gas appliances used. a flow rate measurement unit that measures at given time intervals a flow rate of gas flowing through a passage and that outputs measurement results as flow rate data; a computing unit that calculates differential values from the flow rate data output from the flow rate measurement unit; an appliance determination unit that determines use of an appliance from the differential values computed by the computing unit; and an appliance flow rate calculation unit that calculates a usage flow rate for each appliance, characterized in that the appliance flow rate calculation unit calculates a usage flow rate for each appliance by switching a calculation method in accordance with a result of determination made by the appliance determination unit, the appliance flow rate calculation unit has a plurality of calculation processing units that calculate a flow rate in predetermined steps; and the switching of the calculation processing units is based on: the number of times of flow rate measurements performed by the flow rate measurement unit during a time from when a transition to a state of determination of an appliance is effected and to when processing leaves the state of determination, wherein the state of determination of the appliance starts when the appliance determination unit determines a gas appliance having started operation due to an amount of change of the flow rate of gas coming to a value other than zero and wherein the state of determination of the appliance ends either when the appliance determination unit determines the gas appliance having ended operation due to the amount of change of the flow rate of gas coming to zero again or when a predetermined time has elapsed from the transition to the state of determination of the appliance, wherein the predetermined time means the longest time among times that are required for the appliance determination unit to determine an appliance and that change from one gas appliance to another, and the number of gas appliances used. the number of times of flow rate measurements performed by the flow rate measurement unit during a time from when a transition to a state of determination of an appliance is effected and to when processing leaves the state of determination, wherein the state of determination of the appliance starts when the appliance determination unit determines a gas appliance having started operation due to an amount of change of the flow rate of gas coming to a value other than zero and wherein the state of determination of the appliance ends either when the appliance determination unit determines the gas appliance having ended operation due to the amount of change of the flow rate of gas coming to zero again or when a predetermined time has elapsed from the transition to the state of determination of the appliance, wherein the predetermined time means the longest time among times that are required for the appliance determination unit to determine an appliance and that change from one gas appliance to another, and the number of gas appliances used.