To provide a detector which can simplify its attaching mechanism, can reduce the time required by attaching work and characteristic tests, and can work as a monitor of a centralized environment monitoring system.

The main body section of an ionization chamber is constituted of an external high-voltage electrode 42a, an internal high-voltage electrode 42b, a holding base 43a for both of the electrodes 42a and 42b, a plurality of insulating bases 51, which hold a current collecting electrode 45a in an insulating state, and an insulating base 46a with terminal which fetches the signal current of the electrode 45a. Each electrode is formed in a cylindrical shape having a semispheric top and the center of each semisphere is aligned with that of another semisphere. A scinti-probe 6 is inserted into the high-voltage electrode 42b on the inside in a state where the probe 6 is insulated with an insulating case 63 and positioned so that the detecting center of the ionization chamber may be aligned with that of a scintillator 61.