A gas safety device (100) is provided with: a flow rate measurement unit (103) for measuring the flow rate of gas flowing through a flow path (101); a first pressure sensor that is disposed inside the flow path (101) and measures an absolute pressure of the gas; and a second pressure sensor that is disposed outside the flow path (101) and measures an absolute pressure of the atmospheric pressure. The invention is also provided with: a gas pressure true value acquisition unit (108) for acquiring a true value of gauge pressure of the gas from the exterior; and a gas pressure determination unit (109) for calculating a correction value of a measurement value from the difference between the measurement values of the first pressure sensor and the second pressure sensor and from the true value, and calculating a gas supply pressure on the basis of the difference between the measurement values of the first pressure sensor and the second pressure sensor and on the basis of the correction value. The invention is further provided with: a shut-off valve (102) for shutting off the flow path (101); and a control circuit (104) for controlling the flow rate measurement unit (103) and shutting off the flow path (101) with the shut-off valve (102) if there has been determined to be an abnormality from the flow rate measured at the flow rate measurement unit (103) or the gas supply pressure calculated at the gas pressure determination unit (109).