A gas leak sensing device includes a constant flow rate control valve, a pressurization control valve, a supply-side gas circuit, a master-side gas circuit, a workpiece-side gas circuit, an equal pressure valve, an exhaust valve, a test pressure sensor, and a differential pressure sensor. The supply-side gas circuit is connected with the constant flow rate control valve and the pressurization control valve. The equal pressure valve performs opening and closing between the supply-side gas circuit and the master-side gas circuit and opening and closing between the supply-side gas circuit and the workpiece-side gas circuit. The exhaust valve performs opening and closing between the workpiece-side gas circuit and outside. The test pressure sensor detects pressure in the supply-side gas circuit. The differential pressure sensor detects a differential pressure between the master-side gas circuit and the workpiece-side gas circuit.