A gas shut-off device that monitors usage states of a plurality of gas appliances and shuts off the supply of gas at the time of abnormality occurrence, the gas shut-off 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 the presence or absence of a change in the flow rate of the gas appliances based on the average flow rate obtained by the average flow rate calculation unit; a flow rate storage unit that is configured to store a value of the average flow rate obtained after the decision of the presence of the change decided by the flow rate change decision unit; an increase flow rate registration unit that is configured to register 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 re-registration unit that is configured to reduce 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, and re-register and replace it with a value of the average flow rate obtained by the average flow rate calculation unit after the reduction of the registered flow rate; a monitoring value memory unit that is configured to store a decision value for monitoring the usage states of the gas appliances; an abnormality decision unit that is configured to decide whether or not an abnormality exists from the registered flow rate of the increase flow rate registration unit and the decision value of the monitoring value memory unit; a shut-off unit that is configured to shut off the supply of gas when the abnormality established by the abnormality decision unit exists; and a registration number counting unit that is configured to count a registration number of the registered flow rates, wherein the decrease flow rate re-registration unit re-registers the average flow rate which is obtained by the average flow rate calculation unit as the registered flow rate when the registration number counted by the registration number counting unit is reduced to be one. 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 the presence or absence of a change in the flow rate of the gas appliances based on the average flow rate obtained by the average flow rate calculation unit; a flow rate storage unit that is configured to store a value of the average flow rate obtained after the decision of the presence of the change decided by the flow rate change decision unit; an increase flow rate registration unit that is configured to register 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 re-registration unit that is configured to reduce 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, and re-register and replace it with a value of the average flow rate obtained by the average flow rate calculation unit after the reduction of the registered flow rate; a monitoring value memory unit that is configured to store a decision value for monitoring the usage states of the gas appliances; an abnormality decision unit that is configured to decide whether or not an abnormality exists from the registered flow rate of the increase flow rate registration unit and the decision value of the monitoring value memory unit; a shut-off unit that is configured to shut off the supply of gas when the abnormality established by the abnormality decision unit exists; and a registration number counting unit that is configured to count a registration number of the registered flow rates, wherein the decrease flow rate re-registration unit re-registers the average flow rate which is obtained by the average flow rate calculation unit as the registered flow rate when the registration number counted by the registration number counting unit is reduced to be one.