Abstract: Automatic cutoff and tester apparatus of gas supply is shown which utilizes a diaphragm powered valve, a disk which closes an intermediate opening of a gas pipe, and a gas distributor plenum which is formed by the disk and a diaphragm, and sensor means having several levers and metallic weight which is normally attracted to a permanent magnet means. The gas distributor plenum is passed to gas pipe through bypasses in which a reset valve and a sensor valve of swing are arranged. The sensor means operates automatically in accordance with swing. The diaphragm powered valve cuts off the gas pipe by operation of the diaphragm in accordance with operation of the sensor means, when earthquake occurred.