To provide a spheric ionization chamber detector having an excellent energy characteristic.

A high-voltage electrode 11 is made of a stainless steel sheet having a thickness of 2 mm and a cover 14 is made of another stainless steel having a thickness of 1 mm. Then the surface of the cover 14 is equally divided into spindle-shaped areas as shown in the figure and each divided area is coated with a spindle-shaped lead filter 15 having a thickness of 0.3 mm at a coverage of 50-75%. Therefore, an energy characteristic of ≤±15% can be secured within the range of 80 keV to 3 MeV. It is not necessary to equally divide the surface of the cover 14 nor to make the divided area to have the spindle-like shape. However, it is a necessary condition to cover each divided area with an energy filter (for example, the lead filter) at a prescribed coverage.