PURPOSE: To obtain a detector which provides accurate detection of gas pressure by inspecting an ionizable component by flowing an ion current by applying a voltage between electrodes arranged by ionizing gas irradiated with radiation.

CONSTITUTION: Sensor A, filled with gas, has case 1 with a shield layer shielding radiation. Mesh-shaped electromagnets 2 and 3 are arranged nearly in parallel and radiation source 4 is fitted to electrode 2. As radiation source 4, radium, etc., are used which radiate a great quantity of α rays. Sensor A is further provided with sealed ion chamber 5, electrodes 6 and 7, and radioactivity source 8. External detection circuit B has field effect transistor 9; electrodes 3 and 7 are connected to drain D and source S, and electrodes 2 and 6 to gate G1 respectively. The pointer movement of an ammeter varies with the value of air pressure, so that the gas pressure in case 1 can be detected.