A fire detection device comprising an enclosure enclosing a reference ionization chamber and an unsaturated measuring ionization chamber interconnected in series and each containing a source of radioactive radiations, at least one alarm and/or automatic intervention device, a grid permeable to gases defining one end of the enclosure, the enclosure having apertures communicating with passages at one end of the unsaturated chamber so that gas can flow through the unsaturated chamber, from outside the device, a common electrode extending into both chambers and a printed circuit controlling the alarm and/or intervention device.