PURPOSE: To prevent the variation of the pitch of the joints between electrode blocks owing to the ambient temperature change, by fixing each of the adjacent electrode holding plates of the adjacent electrode blocks together respectively through connecting plates having thermal expansion coefficient nearly equal to that of said holding plates as one body.

CONSTITUTION: Signal electrode plates 11 and high voltage electrode plates 12 are holded alternately between the pairs of holding plates 13 of ceramics or the like to form an electrode block 1a each. These electrode blocks are contained air-tightly within a container 22 of aluminum or the like with the intervention of a bottom plate 16 made from 45% Ni-Fe alloy and thus an ionization chamber type radiation detector is formed. In this case, the upper and lower holding plates 13 of the adjacent electrode blocks 1a are fixed respectively by connecting plates 24 of 45% Ni-Fe alloy having thermal expansion coefficient nearly equal to that of the holding plates 13 as one body. Therefore, it is possible to absorb the variation of the pitch of the joints between the electrode blocks owing to the ambient temperature change and to assure accurate performance of the detector.