A gas cutoff device that monitors using states of a plurality of gas appliances and cuts off the supply of gas at the time of abnormality occurrence, the gas cutoff device comprising: a flow rate detection unit that is configured to measure a flow rate; a flow rate calculation unit that is configured to calculate values of instant flow rates from a detection value of the flow rate detection unit; an average flow rate calculation unit that is configured to obtain an average flow rate from the instant flow rates obtained by the flow rate calculation unit; a flow rate change decision unit that is configured to decide whether or not an appliance change flow rate appears based on the obtained average flow rate; an increase flow rate registration unit that is configured to register, as a registered flow rate, an amount of the flow rate change at the time of increase change decision by the flow rate change decision unit; a decrease flow rate registration unit that is configured to reduce and correct the registered flow rate near an amount of the flow rate change at the time of decrease change decision by the flow rate change decision unit; a registered flow rate summarization unit that is configured to calculate a total value of the registered flow rate; a flow rate difference decision unit that is configured to calculate a flow rate difference from the average flow rate and the registration total flow rate, to cause the increase flow rate registration unit to register the flow rate difference when the flow rate difference is positive, and to cause the decrease flow rate registration unit to perform a flow rate output which reduces and corrects the registered flow rate from a low level when the flow rate difference is negative; an monitoring value memory unit that is configured to store a decision value for monitoring usage states of the appliances; an abnormality decision unit that is configured to decide whether or not an abnormality exists from the registered flow rate of the flow rate registration unit and the decision value of the monitoring value memory unit; and a cutoff unit that is configured to cut off the supply of gas at the time of an abnormality decision established by the abnormality decision unit. a flow rate detection unit that is configured to measure a flow rate; a flow rate calculation unit that is configured to calculate values of instant flow rates from a detection value of the flow rate detection unit; an average flow rate calculation unit that is configured to obtain an average flow rate from the instant flow rates obtained by the flow rate calculation unit; a flow rate change decision unit that is configured to decide whether or not an appliance change flow rate appears based on the obtained average flow rate; an increase flow rate registration unit that is configured to register, as a registered flow rate, an amount of the flow rate change at the time of increase change decision by the flow rate change decision unit; a decrease flow rate registration unit that is configured to reduce and correct the registered flow rate near an amount of the flow rate change at the time of decrease change decision by the flow rate change decision unit; a registered flow rate summarization unit that is configured to calculate a total value of the registered flow rate; a flow rate difference decision unit that is configured to calculate a flow rate difference from the average flow rate and the registration total flow rate, to cause the increase flow rate registration unit to register the flow rate difference when the flow rate difference is positive, and to cause the decrease flow rate registration unit to perform a flow rate output which reduces and corrects the registered flow rate from a low level when the flow rate difference is negative; an monitoring value memory unit that is configured to store a decision value for monitoring usage states of the appliances; an abnormality decision unit that is configured to decide whether or not an abnormality exists from the registered flow rate of the flow rate registration unit and the decision value of the monitoring value memory unit; and a cutoff unit that is configured to cut off the supply of gas at the time of an abnormality decision established by the abnormality decision unit.