A gas shutoff apparatus, comprising: a flow velocity detection unit which detects a flow velocity by measuring a signal transfer time in a medium; a flow volume calculation unit which converts the flow velocity detected by the flow velocity detection unit into a flow volume; an abnormality determination unit; and a shutoff unit adapted to shut off a fluid path through which the medium flows; a return unit which outputs a return signal to the shutoff unit to open the fluid path; characterized in that a) the gas shutoff apparatus shuts off the gas flow in the case of an abnormality caused by abnormal amount of gas flux, b) the abnormality determination unit determines whether or not the flow volume calculated by the flow volume calculation unit is normal; c) the shutoff unit shuts off a fluid path through which the medium flows when the abnormality determination unit determines that flow volume calculated by the flow volume calculation unit is abnormal, d) the return unit outputs the return signal, when the abnormality determination unit has determined that the flow volume calculated is abnormal and in response thereto the shutoff unit has shut off the fluid path, and that e) the gas shutoff apparatus comprises a return time-counting unit which starts a time-counting on the basis of a return operation of the return unit; and a leakage determination unit which outputs a driving signal to the shutoff unit to shutoff the fluid path when the flow volume calculation unit detects that a flow volume is equal to or greater than a predetermined flow volume after the return time-counting unit counts that a predetermined time period has been elapsed. a flow velocity detection unit which detects a flow velocity by measuring a signal transfer time in a medium; a flow volume calculation unit which converts the flow velocity detected by the flow velocity detection unit into a flow volume; an abnormality determination unit; and a shutoff unit adapted to shut off a fluid path through which the medium flows; a return unit which outputs a return signal to the shutoff unit to open the fluid path; a) the gas shutoff apparatus shuts off the gas flow in the case of an abnormality caused by abnormal amount of gas flux, b) the abnormality determination unit determines whether or not the flow volume calculated by the flow volume calculation unit is normal; c) the shutoff unit shuts off a fluid path through which the medium flows when the abnormality determination unit determines that flow volume calculated by the flow volume calculation unit is abnormal, d) the return unit outputs the return signal, when the abnormality determination unit has determined that the flow volume calculated is abnormal and in response thereto the shutoff unit has shut off the fluid path, and that e) the gas shutoff apparatus comprises a return time-counting unit which starts a time-counting on the basis of a return operation of the return unit; and a leakage determination unit which outputs a driving signal to the shutoff unit to shutoff the fluid path when the flow volume calculation unit detects that a flow volume is equal to or greater than a predetermined flow volume after the return time-counting unit counts that a predetermined time period has been elapsed.