Ionization type-smoke detector

An ionization-type smoke detector having inner and outer ionization chambers, a radioactive source to ionize the air within said inner and outer ionization chambers, electrical circuit elements to detect electrical potential at a connection point between said inner and outer ionization chambers, and a printed board mounted horizontally in said detector to support said electrical circuit elements, said radioactive source producing radiation in a direction parallel to said printed board to ionize the air within said inner and outer ionization chambers, plate-like outer, intermediate and inner electrodes respectively mounted to and extending vertically from said printed board at fixed intervals to form said inner ionization chamber between said inner and intermediate electrodes, and to form said outer ionization chamber between said outer and intermediate electrodes, said inner electrode supporting said radioactive source, said intermediate electrode being formed with a through-hole such that said radioactive source can irradiate into said outer ionization chamber through said through-hole to ionize the air within said outer ionization chamber and, said outer electrode shielding at least those of said electrical circuit elements which must be electrically shielded.