With a gas shutoff device in the related art, a non-uniform gas layer can be produced by mixing of a gas of a different type from the gas currently in use during a shift in the amount of heat or installation or replacement, and an unexpected flow rate may be detected due to a disruption in the propagation of ultrasonic waves, thereby causing an erroneous determination of a flow rate abnormality or a sensor abnormality. With the gas shutoff device according to the invention, a valve-closing timer section 14 starts to time based on a valve-closing signal which is output by the flow rate abnormality or the sensor abnormality, and outputs a valve-opening signal to a valve driving section 12 if a release input is accepted from outside within a setting time period. Thus, even when the non-uniform gas layer is produced by mixing the gas of a different type from the gas currently in use during the shift in the amount of heat or installation or replacement, an unexpected flow rate is detected due to the disruption in the propagation of ultrasonic waves, thereby causing the erroneous determination of the flow rate abnormality or the sensor abnormality, and the valve is closed, the erroneous determination is resolved and the gas is available again within a given time period.